How do I get the imessages on my iphone to transfer to my ipad?

1. Set up text message forwarding
   1. On your iPhone, iPad, or iPod touch, go to Settings > Messages > Send & Receive. Make sure that you're signed in to iMessage with the same Apple ID on all your devices.
   2. On your iPhone, go to Settings > Messages > Text Message Forwarding.*
   3. 
   4. Choose which devices can send and receive text messages from your iPhone. If you're not using two-factor authentication for your Apple ID, a verification code appears on each of your other devices: enter that code on your iPhone.
   5. 

Make sure both devices are logged in to the same Apple ID.

Then on your iPhone, go to Settings ➜ Messages ➜ Text Message Forward and make sure it's turned on, and your iPad is turned on there as well.

click the following link for more details. Link ➡️ Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support