Return page but leave input

I am creating a simple login form. upon button click this function is fired:

public async Task<IActionResult> OnPostButton_Login(IFormCollection data)
{
    var emailAddress = Request.Form["emailaddress"];     
    var pass = Request.Form["password"];

    if (String.IsNullOrEmpty(emailAddress)) return Page();
    if (String.IsNullOrEmpty(pass)) return Page();

    await GetUser(emailAddress , pass);

    return Page();
}

If the user doesn't type anything under either email or password, the page reloads - but also clears all inputs.

How can I return from this function but leave everything untouched (and just return an alert?)

> How can I return from this function but leave everything untouched (and just return an alert?)

Add the [BindProperty] attribute to the properties you want to reload into the returned page.

[BindProperty]
public string emailaddress { get; set; }

[BindProperty]
public string password { get; set; }

Use the ModelState.AddModelError method to add a string alert/message to the returned page.

ModelState.AddModelError(string.Empty, "My message: Please provide an email address.");

Here's a stripped down version of a login page using your code:

Login.cshtml.cs

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;

namespace WebApplication1.Areas.Identity.Pages.Account
{
    public class LoginModel : PageModel
    {
    [BindProperty]
    public string emailaddress { get; set; }

    [BindProperty]
    public string password { get; set; }

        public async Task OnGetAsync(string returnUrl = null)
        {
            await HttpContext.SignOutAsync(IdentityConstants.ExternalScheme);
        }

        public async Task<IActionResult> OnPostButton_Login(IFormCollection data)
        {
            string emailAddress = Request.Form["emailaddress"].ToString();
            string pass = Request.Form["password"].ToString();

            if (String.IsNullOrEmpty(emailAddress))
            {
                ModelState.AddModelError(string.Empty, "My message: Please provide an email address.");
                return Page();
            }
            if (String.IsNullOrEmpty(pass))
            {
                ModelState.AddModelError(string.Empty, "My message: Please provide a password.");
                return Page();
            }

            await GetUser(emailAddress, pass);

            return Page();
        }

        private Task GetUser(string emailAddress, string pass)
        {
            throw new NotImplementedException();
        }
    }
}

Login.cshtml

@page
@model LoginModel
@{
    ViewData["Title"] = "Log in";
}
<h1>@ViewData["Title"]</h1>
<div class="col-md-4">
    <section>
        <form id="account" method="post">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-floating">
                <input asp-for="emailaddress" class="form-control" autocomplete="username" aria-required="true" />
                <label asp-for="emailaddress" class="form-label"></label>
                <span asp-validation-for="emailaddress" class="text-danger"></span>
            </div>
            <div class="form-floating">
                <input asp-for="password" class="form-control" autocomplete="current-password" aria-required="true" />
                <label asp-for="password" class="form-label"></label>
                <span asp-validation-for="password" class="text-danger"></span>
            </div>
            <div>
                <input type="submit" value="Submit" asp-page-handler="Button_Login" />
            </div>
        </form>
    </section>
</div>

Alternative, manual control of the submit process

Here's a full list of what can be returned by a post method in a Razor page. You can take control of the submit process through JavaScript on the client and using the fetch API.

You can post to the OnPostButton_Login and then return with a content result (e.g. return Content("This is a message."); to the Promise callback of the fetch method. In this process, your page does not reload but just the input values in the form are posted. Here's a SO post of controlling the submit process.