How to add options dynamically to Dropdown fields in CRM

How to add options dynamically to Dropdown fields in CRM

I have a dropdown field in CRM.

OnLoad with client script, I want to dynamically fill this dropdown with some values dynamically.
How would i do this?

The setValue() method only sets a value that already exists, but doesnt create a new one.
Could I use ui.add somehow?

This has to happen when the user edits a record in CRM.