I found this on Reddit, and it worked perfectly for my Apple-unlocked iPhone 13 Pro. Hope it helps you too…
This guide has been tested with an Unlocked (purchased directly from Apple.com) iPhone 13 Pro Max.
First, locate your IMEI2 number on the NEW device and take note of it. This is found Settings > General > About > Scroll to bottom of page.
Second, login into your VZW “online” account. (This method was not tested via the mobile app, but in Safari). Navigate to:
Select the three Lines at the top right hand corner of the window
Select “My devices”
Select “Activate or Switch device”
Select “Activate on an existing line”
Select “Your eligible device (iPhone/Android device)”
Answer the prompted questions to the best of your knowledge
You should come to a screen asking for you to enter an “Device ID IMEI”. Input the IMEI2 Number found in your new iPhone 13.
Continue through the prompts, I recommend not adding Verizon coverage.
Once you complete all the steps your NEW iPhone 13 will have a screen asking to activate.
Follow the steps to configure your eSIM for personal, business, etc. use.
The activation can take 1-3min, be patient 🙂