This should get the id added.
ASP.NET MVC 5 and lower:
<% using (Html.BeginForm(null, null, FormMethod.Post, new { id = "signupform" }))
{ } %>
ASP.NET Core: You can use tag helpers in forms to avoid the odd syntax for setting the id.
<form asp-controller="Account" asp-action="Register" method="post" id="signupform" role="form"></form>