antiforgery stopping basic razor page post with 400 error - asp.net-core

Question

I have been getting a 400 error, and cut the code down to a very basic form post, and still get this error.
It happens once only on first attempt (i.e. first time page is loaded ever, then, go back, press button, and all ok)
I have found that it is related to "antiforgery", and if i disable this, all works perfect first time
<div class="text-center">
<form method="post">
<button id="btnLogin" type="submit" asp-page-handler="BtnSignin">Login</button>
</form>
The above is my HTML, and below is the c#:
public async Task<IActionResult> OnPostBtnSignin()
{
return null;
}
(i know returning null isnt ideal, but, for my debugging, it confirms if i get the 400 error or not)
if I publish (to aws lambda), on first button press, it errors, go back, press button, and this time, no error
if i disable "antiforgery", it works (have tried, and its consistent either way round)
Any ideas on why? and how to fix? (without disabling antiforgery)

Answer 1

Try to add #Html.AntiForgeryToken() to your form:
<form method="post">
#Html.AntiForgeryToken()
<button id="btnLogin" type="submit" asp-page-handler="BtnSignin">Login</button>
</form>
Both <form method="post"></form> and #Html.AntiForgeryToken() will result in the __RequestVerificationToken hidden field being added to the page.So if your form cannot add __RequestVerificationToken to page,you can try to add #Html.AntiForgeryToken().