Enabling drop-down menus with developer mode

How do you use the Input Slot Options feature? I wish to make a constant block to allow users get constants such as pi, golden ratio, infinity, e, and so on. Is there a way to inspect existing blocks to see how those features are implemented?

I got the drop down part working but I do not know how to make it look the same.

Nevermind, got it. Have to make it readonly and the menus work like this