Selecting from drop down list to populate a new field

Hi WordPress community. I’m back after about a 1 year absence. I’m creating a new site and having trouble with this.

I want to have a drop down list of my categories so when you select a category, the name of the selected category will populate a separate field.

Any one know how to do it?

Might need to write a function for it first – not sure.

A general outline would be ok to start and then I possibly could tweek it.

I’ll be back tonight to check.

Thanks for your help.