[asp.net] How to POST a FORM from HTML to ASPX page

This is very possible. I mocked up 3 pages which should give you a proof of concept:

.aspx page:

<form id="form1" runat="server">
    <div>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:TextBox TextMode="password" ID="TextBox2" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" Text="Button" />
    </div>
</form>

code behind:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    For Each s As String In Request.Form.AllKeys
        Response.Write(s & ": " & Request.Form(s) & "<br />")
    Next
End Sub

Separate HTML page:

<form action="http://localhost/MyTestApp/Default.aspx" method="post">
    <input name="TextBox1" type="text" value="" id="TextBox1" />
    <input name="TextBox2" type="password" id="TextBox2" />
    <input type="submit" name="Button1" value="Button" id="Button1" />
</form>

...and it regurgitates the form values as expected. If this isn't working, as others suggested, use a traffic analysis tool (fiddler, ethereal), because something probably isn't going where you're expecting.