How to perform a search inside a specific taxonomy category

Hello everyone.
My problem is this:
I have 3 custom taxonomies, let’s say ‘author’, ‘title’ and ‘editor’, each one applied to regular posts.
Let’s say I have post_1 that has in the ‘author’ field ‘jorge borges’ and post_2 that has ‘ray bradbury’.

I’m trying to have a search form with a drop down menu containing the three taxonomies and a text field, so that if I select, i.e., ‘author’ and search for ‘jorge borges’, the result will be post_1.

The other two taxonomies should work like this as well.

I couldn’t find anything similar, as many questions concern the creation of a drop down menu with all instances of a taxonomy, which is not what I want.
I want a drop down menu with taxonomy categories, not values.

Thanks in advance for all the help…