createUiDefinition.json for ARM Azure web app

I need to validate the azure web app site name is already exists or not.

This is not possible, you need to add to the site name a unique String, to ensure that the site name is globally unique. For example you could use in your ARM template this function: uniqueString()

Similar question was answered by a Microsoft employee.

Also need to create or use the app service plan.

Add an App Service Plan to your Azure Resource Manager Template. For example like this:

{
  "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "hostingPlanName": {
      "type": "string",
      "minLength": 1
    },
    "skuName": {
      "type": "string",
      "defaultValue": "F1",
      "allowedValues": [
        "F1",
        "D1",
        "B1",
        "B2",
        "B3",
        "S1",
        "S2",
        "S3",
        "P1",
        "P2",
        "P3",
        "P4"
      ],
      "metadata": {
        "description": "Describes plan's pricing tier and capacity. Check details at https://azure.microsoft.com/en-us/pricing/details/app-service/"
      }
    },
    "skuCapacity": {
      "type": "int",
      "defaultValue": 1,
      "minValue": 1,
      "metadata": {
        "description": "Describes plan's instance count"
      }
    }
  },
  "variables": {
    "webSiteName": "[concat('webSite', uniqueString(resourceGroup().id))]"
  },
  "resources": [
    {
      "apiVersion": "2015-08-01",
      "name": "[parameters('hostingPlanName')]",
      "type": "Microsoft.Web/serverfarms",
      "location": "[resourceGroup().location]",
      "tags": {
        "displayName": "HostingPlan"
      },
      "sku": {
        "name": "[parameters('skuName')]",
        "capacity": "[parameters('skuCapacity')]"
      },
      "properties": {
        "name": "[parameters('hostingPlanName')]"
      }
    },
    {
      "apiVersion": "2015-08-01",
      "name": "[variables('webSiteName')]",
      "type": "Microsoft.Web/sites",
      "location": "[resourceGroup().location]",
      "tags": {
        "[concat('hidden-related:', resourceGroup().id, '/providers/Microsoft.Web/serverfarms/', parameters('hostingPlanName'))]": "Resource",
        "displayName": "Website"
      },
      "dependsOn": [
        "[resourceId('Microsoft.Web/serverfarms/', parameters('hostingPlanName'))]"
      ],
      "properties": {
        "name": "[variables('webSiteName')]",
        "serverFarmId": "[resourceId('Microsoft.Web/serverfarms', parameters('hostingPlanName'))]"
      }
    }
  ]
}

Validating an app service's name is possible in createUiDefinition.json.

The crux of it is the Microsoft.Solutions.ArmApiControl, which can be used to call ARM apis as part of the validation of a text box. Here is an example:

    {
      "$schema": "https://schema.management.azure.com/schemas/0.1.2-preview/CreateUIDefinition.MultiVm.json#",
      "handler": "Microsoft.Azure.CreateUIDef",
      "version": "0.1.2-preview",
      "parameters": {
        "basics": [
          {}
        ],
        "steps": [
          {
            "name": "domain",
            "label": "Domain Names",
            "elements": [
              {
                  "name": "domainInfo",
                  "type": "Microsoft.Common.InfoBox",
                  "visible": true,
                  "options": {
                      "icon": "Info",
                      "text": "Pick the domain name that you want to use for your app."
                  }
              },
              {
                "name": "appServiceAvailabilityApi",
                "type": "Microsoft.Solutions.ArmApiControl",
                "request": {
                  "method": "POST",
                  "path": "[concat(subscription().id, '/providers/Microsoft.Web/checknameavailability?api-version=2021-02-01')]",
                  "body": "[parse(concat('{\"name\":\"', concat('', steps('domain').domainName), '\", \"type\": \"Microsoft.Web/sites\"}'))]"
                }
              },
              {
                "name": "domainName",
                "type": "Microsoft.Common.TextBox",
                "label": "Domain Name Word",
                "toolTip": "The name of your app service",
                "placeholder": "yourcompanyname",
                "constraints": {
                  "validations": [
                    {
                      "regex": "^[a-zA-Z0-9]{4,30}$",
                      "message": "Alphanumeric, between 4 and 30 characters."
                    },
                    {
                      "isValid": "[not(equals(steps('domain').appServiceAvailabilityApi.nameAvailable, false))]",
                      "message": "[concat('Error with the url: ', steps('domain').domainName, '. Reason: ', steps('domain').appServiceAvailabilityApi.reason)]"
                    },
                    {
                      "isValid": "[greater(length(steps('domain').domainName), 4)]",
                      "message": "The unique domain suffix should be longer than 4 characters."
                    },
                    {
                      "isValid": "[less(length(steps('domain').domainName), 30)]",
                      "message": "The unique domain suffix should be shorter than 30 characters."
       

             }
              ]
            }
          },
          {
            "name": "section1",
            "type": "Microsoft.Common.Section",
            "label": "URLs to be created:",
            "elements": [
              {
                "name": "domainExamplePortal",
                "type": "Microsoft.Common.TextBlock",
                "visible": true,
                "options": {
                    "text": "[concat('https://', steps('domain').domainName, '.azurewebsites.net - The main app service URL')]"
                }
              }
            ],
            "visible": true
          }
        ]
      }
    ],
    "outputs": {
      "desiredDomainName": "[steps('domain').domainName]"
    }
  }
}

You can paste that in to the createUiDefinition.json sandbox azure provides to test it out.

To look at how to do an Azure App Service instead of a VM, look in to documentation like this and use it as the maintemplate.json in the app.zip package.