Hello, I am trying to create a field in my Purchase Orders page that looks up a value from another table, based on the Vendor No. field, like below.

(Purchase Order page, with Payment Terms Code looked up from the Vendor table. It should be Read-Only.)
I haven't been able to figure out how to accomplish this, and my googling has been unfruitful (not sure what it's called). I looked up the base application code for Vendor Name and No., but it hasn't helped.

Any advice? Thank you.