requester

noun

Etymology: From request + -er; compare requestor, requestress.

One who, or that which, makes a request.
A dialog box that prompts the user to choose from a list.
“Clicking on Add True-Type brings up an OS-specific font requester. Highlighting a font and clicking OK returns you to the Edit Font List window, now with the selected font appearing in the Font pop-up menu.”
“File requesters, help files, configuration utilities, and menus would all be different […]”