ASP.NET Core MVC pass a url parameter to the POST

I have a Blazor Server app. It uses the ASP.NET Identity Library which is ASP.NET Core MVC. I am trying to pass a parameter to register so a URL like /identity/account/Register?follow=uniqueId gives me the parameter follow=uniqueId.

In Register.cshtml.cs (class RegisterModel) I have:

public async Task OnGetAsync(string returnUrl = null, string following = null)
{
    ReturnUrl = returnUrl;
    Following = following;
    ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();
}

public async Task<IActionResult> OnPostAsync(string returnUrl = null, string following = null)
{
    // ...
}

The value from the url is passed in to OnGetAsync() fine. But when OnPostAsync() is called, it passes in a null for following and the object property Following is also null.

How do I get that parameter in OnPostAsync()?

According to your description, the reason why the following is always null is you don't set the [FromQuery] attribute and not use the right query string.

Inside the url your querystring is follow not following which will not bind well.

I suggest you could modify the method like below:

public async Task<IActionResult> OnPostAsync(string returnUrl = null, [FromQuery]string following = null)
{
    // ...
}

---

Update: As you said, POST requests do not automatically include URL parameters unless you set it inside the from action attribute.

Since you don't share how you post the request, I provide two way which could make this post work well.

More details, you could refer to below example:

1.You set the follow query string inside the form tag action like below:

<form action="/register?follow=uniqueId" method="post">

    <input type="submit" value="Submut"/>
    

</form>

2.If you are using the razor page and still want to use it, you could refer to below :

<form asp-route-returnUrl="@Model.ReturnUrl" method="post" asp-route-follow="@Model.Follow">

3.If you are using the razor page and post it inside the razor page not blazor, you could use hidden field and use the bind parameter like below:

<form method="post">

    <input type="hidden" asp-for="Following"  value="test"/>
    <input type="submit" value="Submut" />


</form>

Inside the razor page.cs add below codes:

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