Hello Mike,
Thank you for using Apple Support Communities.
That is just a simple setting in the Messages preferences. Take a look at this article:
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support
If you look at the section under "Use iMessage and FaceTime with multiple devices", see the following screen from Settings > Messages.
Check the phone number under "Start New Conversations From".
If you don't see the phone number there, you will need to look further down to the section "Phone number missing on iPad or iPod touch".
Cheers,
Jeff D.