I ended up finding the solution on some other forums, but I made some improvements and I wanted to put it all together in one place for everyone.
add the following script to the top of
..\admin\Security\Signin.ascx.vb
<script runat="server">
Sub change(ByVal sender As Object, ByVal e As EventArgs)
Select Case txtUsername.Text
Case "admin"
Case "host"
Case Else
If txtUsername.Text.StartsWith("Domain") Then
'The user added the wrong domain information
If txtUsername.Text.Contains("/") Then
txtUsername.Text = txtUsername.Text.Replace("/", "\")
End If
Else
'The user didn't enter the domain so add it for them
txtUsername.Text = String.Format("Domain\{0}", txtUsername.Text)
End If
End Select
End Sub
</script>
Then change
<td colspan="2" align="center"><asp:textbox id="txtUsername" columns="9" width="130" cssclass="NormalTextBox" runat="server" /></td>
to:
<td colspan="2" align="center"><asp:textbox id="txtUsername" ontextchanged="change" columns="9" width="130" cssclass="NormalTextBox" runat="server" /></td>