use a dropdown to populate text in another field in a WEB form

New user here!

I have created a form where I have a drop down (e.g. list of options where different rules apply to each) and depending on the option picked, different text would populate in another field (e.g. to identify which rules need to be applied). I used a custom validation script ( if/else script ) in the dropdown to achieve this. I am now wanting to do this same action but in a web form. Is this possible/ how should I go about achieving this? Any help appreciated