Brand new to JS.

Working in Adobe, attempting to take a drop down field and based on the value selected I want to use a 3 digit # into a text field that is beside it. (it will be read only)

I do not have a clue how to start this and I am not sure how to search for help on it since I am not clear what I am asking for?

Anyone willing to point me in the right direction or explain how this script should look?

Thank you in advance.