Concatenate Textbox With Email Address

I have an asp page and wish to concatenate a Contact Name textbox to part of an email
address and place this concatenated text into an email textbox. I know how to concatenate
but unsure of how to implement this via the textbox event, onexit. Can this be done?

The code I'm using is:

Dim EnterName
Dim NewEmail
EnterName = Request.Form("txtContactName")
If Instr(EnterName," ") <> 0 Then
EnterName = Replace(EnterName," ",".")
NewEmail = EnterName & "@something.com"
End If

The html code for the email textbox is:

<Input type="text" Name="txtEmailAddress" value="<%=NewEmail%>>

Like I said not sure if this will not work until the txtContactName has an OnExit event, or something similar.