Dialog box is not opening/application freezes while calling a API in tkinter

you need to do any work in a different thread, however all gui updates should happen in the main thread or things can get a bit wonky

something like the following untested code should work (maybe with minor alterations)

def do_login(username,password,callback):
    # this should run in a thread so it does not block the gui
    # since it is run in a thread it SHOULD NOT update the gui
    payload = json.dumps({"username": username, "password": password})
    headers = {'Content-Type': 'application/json'}
    callback(requests.post(url+"/users/getuser", data=payload, headers=headers))

import threading
def login_button_fuction():
    # this is called by the gui when the button is clicked
    top = tk.Toplevel(self)
    top.title("loading")
    top.geometry("200x100")
    top.resizable(0, 0)
    bar = ttk.Progressbar(top, orient="horizontal",
                                  length=100, mode="indeterminate")
    bar.pack(pady=25)
    def on_response(response):
        # after the call completes it will call back
        # to this function, once again in the main thread
        # since this is run in the main thread you CAN update the gui            
        print("Got response... hide bar...or something",data)
        if response.json()["resultCode"] == 100:
            print("success")
            data = {
        "realname": response.json()["data"]["realname"],
        "phone": response.json()["data"]["phone"]
            }
            with open("user_data.dat", "wb") as f:
                pickle.dump(data, f)
            
            controller.show_frame(dashboardPage.dashboard_page)

        else:
            print("error")
        bar.grid_forget()
        top.destroy()
        # End callback function

    bar.start()
    # callback needs to run in the main thread
    # root.after does not accept args so it needs to be another lambda
    # that calls the main thread ... there may be a more elegant way to do this
    callback = lambda data:root.after(1,lambda:on_response(data))
    # trigger a new thread to actually do the fetch
    threading.Thread(target=do_login,args=[user_entry.get(),pass_entry.get(),callback]).start()

See additional information about root.after

some additional answer specifically about root.after triggering main thread

below you can find a 100% verified working example(you can just copy/paste) with some extra prints to demonstrate "thready-ness"

import functools
import threading
import time
import requests
import tkinter as tk
from tkinter import ttk

root = tk.Tk()
root.geometry('400x150')
root.title('Tkinter Login Form - pythonexamples.org')

# username label and text entry box
usernameLabel = ttk.Label(root, text="User Name").grid(row=0, column=0)
username = tk.StringVar()
usernameEntry = ttk.Entry(root, textvariable=username).grid(row=0, column=1)

# password label and password entry box
passwordLabel = ttk.Label(root, text="Password").grid(row=1, column=0)
password = tk.StringVar()
passwordEntry = ttk.Entry(root, textvariable=password, show='*').grid(row=1, column=1)

labelVar = tk.StringVar(value="Waiting For Event")
label = ttk.Label(root, textvariable=labelVar).grid(row=10,column=0)
label = ttk.Label(root, text="Valid:'eve.holt@reqres.in'").grid(row=0,column=2)
def do_actual_login(username,password,callback):
    print("current thread(doing login request):", threading.current_thread())
    url="https://reqres.in/api/login"
    data={"username":username,"password":password}
    # fake delay to show not blocking...(request is pretty fast)
    time.sleep(5)
    print("ok really do request in thread")
    return callback(requests.post(url,data))

def validateLogin(username, password):
    print("current thread(on click handler):", threading.current_thread())
    bar = ttk.Progressbar(root, orient="horizontal",
                          length=100, mode="indeterminate")
    bar.grid(row=10,column=0)
    bar.start()

    def login_complete(response):
        print("current thread(login_complete):",threading.current_thread())
        bar.grid_remove()
        if response.status_code == 200:
           print("LOGIN SUCCESS!!!")
           labelVar.set("Login Success")
        else:
           print("LOGIN FAILED:",response,response.json())
           labelVar.set(response.json().get("error","Unknown Error Occurred"))
    # setup our thread and callback
    callback = lambda data:root.after(1,lambda:login_complete(data))
    threading.Thread(
        target = do_actual_login,
        args = [username.get(),password.get(),callback]
    ).start()



validateLogin = functools.partial(validateLogin, username, password)

# login button
loginButton = ttk.Button(root, text="Login", command=validateLogin).grid(row=4, column=0)

root.mainloop()

you can test a successfull login by using the username eve.holt@reqres.in with any password you want (I think) anyother username should fail as should omitting the password