Hi tanws8,
Use BindProperty attribute. Refer below article for more details.
Please refer below sample.
Razor PageModel (Code-Behind) 
public class IndexModel : PageModel
{
    [BindProperty]
    public Guid guid_GroupID { get; set; }
    [BindProperty]
    public string str_is2FA { get; set; }
    public Guid Prop1 { get; set; }
    public string Prop2 { get; set; }
    public void OnGet()
    {
        str_is2FA = "2FA";
        guid_GroupID = Guid.NewGuid();
    }
    public void OnPostDownload()
    {
        Prop1 = guid_GroupID;
        Prop2 = str_is2FA;
    }
}
Razor Page (HTML)
@page
@model VariablesGiveNull.Pages.IndexModel
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@{
}
<!DOCTYPE html>
<html>
<head>
    <meta name="viewport" content="width-device-width" />
    <title>Index</title>
    <style type="text/css">
        body { font-family: Arial; font-size: 10pt; }
    </style>
</head>
<body>
    <form method="post">
        <input type="hidden" asp-for="str_is2FA" />
        <input type="hidden" asp-for="guid_GroupID" />
        <input type="submit" value="Download" asp-page-handler="Download" />
        <hr />
        <label>@Model.Prop2</label>
        <br />
        <label>@Model.Prop1</label>
    </form>
</body>
</html>
Screenshot
