How to Activate Text Message Forwarding on iOS
Step 1. First go to iPhone Settings > Messages > Send & Receive->Add an Email.
Step 2. Enter your Apple ID user name and password. Now you can select email to send and receive Messages.
Step 3. Once the email id is configured, Go back to Message settings.
Step 4. Tap on Text Message Forwarding-> and choose the Mac or iPad.
Step 5. Now go on your Mac or iPad and you’ll see a popup of one-time verification code.
Step 6. All you have to do is enter the code into your iPhone one time.