Settings>Messages>Send and Receive at>You can be reached by iMessages at. Your phone number should be checked as well as your Apple ID email address.
Actually, I assume that you do have this set up correctly, but ti can;'t hurt to check and I don't know what you are not seeing the code on the iPad. It appeared in the Messages app for me when I set it up. It had me stumped for about half an hour because I had no idea where it would be and the Appler support article was not very as to where you can find it.
Connect your iPhone, iPad, iPod touch, and Mac using Continuity - Apple Support
- Your iPhone, iPad, and iPod touch need to use iOS 8.1, and your Mac needs to use OS X Yosemite.
- Sign in to iMessage on your iPhone, your other iOS devices, and your Mac using the same Apple ID.
- On your iPhone:
- Go to Settings > Messages > Send & Receive > You Can Be Reached By, and add a check to both your phone number and email address.
- Go to Messages > Text Message Forwarding, and enable the device(s) you would like to forward messages to.
- Your Mac, iPad, or iPod touch will display a code. Enter this code on your iPhone to verify the SMS feature.
I will be damned if I can see anywhere in this blurb where the code should appear. I made the type bold in the last bullet point above. It was not showing up on my home screen IIRC.
Where is the code supposed to appear Apple????