how to auto select an input field and the text in it on page load

From http://www.codeave.com/javascript/code.asp?u_log=7004:

var input = document.getElementById('myTextInput');
input.focus();
input.select();

<input id="myTextInput" value="Hello world!" />

To do it on page load:

window.onload = function () {
  var input = document.getElementById('myTextInput');
  input.focus();
  input.select();
}

<input id="myTextInput" value="Hello world!" />

In your input tag use autofocus like this

<input type="text" autofocus>

    var input = document.getElementById('myTextInput');
    input.focus();
    input.setSelectionRange( 6,  19 );

    <input id="myTextInput" value="Hello default value world!" />

select particular text on textfield

Also you can use like

input.selectionStart = 6;
input.selectionEnd = 19;

Using the autofocus attribute works well with text input and checkboxes.

<input type="text" name="foo" value="boo" autofocus="autofocus"> FooBoo
<input type="checkbox" name="foo" value="boo" autofocus="autofocus"> FooBoo

Let the input text field automatically get focus when the page loads:

<form action="/action_page.php">
  <input type="text" id="fname" name="fname" autofocus>
  <input type="submit">
</form>

Source : https://www.w3schools.com/tags/att_input_autofocus.asp