Hello Dear

What I understood from your question and correct me if I'm wrong is that you want to add drop down field in website form

There are two ways to this: 

1- The first one is by adding selection field and adding the options to it, select any field in the form and the form customize will open on the right side from there you can add a new field

after that you change the type of the field to selections and you can add, edit, reorder and remove the options.

2- The second way is by selecting an existing field from the backend which type is selection or relationship

You can see in the picture below that if you scrolled down types of the fields you will reach a section of existing field you can then choose the desired field