When you click the button, the e-mail address (in your case) is simply the concatenation of the two fields plus the rest of the address. So assuming the first and last names were in textboxes called, e.g. txtFirstName & txtLastName, then put this code somewhere on the button's click event code.
Code:
Dim strEmail as String, strFirstName as String, strLastName as String
strFirstName = Trim(Nz(Me.txtFirstName, ""))
strLastName = Trim(Nz(Me.txtLastName, ""))
If Len(strFirstName) = 0 Or Len(strLastName) = 0 Then
MsgBox "User Name is not properly entered.", vbCritical, "Invalid Name!"
Else
strEmail = strFirstName & "." & strLastName & "@soandso.com"
End If