There are a couple of ways to create a dropdown in Jenkins.

You can use dropdownDescriptorSelector to create a fully data-bound form and the option to include a config page for the descriptor by creating a file called config.jelly.

If you want a simpler select to be rendered you can use the f:select Jelly tag and create a doFillFieldNameItems method in the descriptor.

Dynamic select

Updates the contents of a "select" control dynamically based on selections of other controls.