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.