Instead of using Templater’s prompt dialog, you’ll want to use the suggester one instead.
Suggester even has a built in way of displaying files within your vault. Here’s an example from the Templater Discussion board about filtering the file selection to only include files from a single directory.
As a warning, you will have to use TemplaterJS instead of just the regular Templater syntax which will run JavaScript on your vault.
Be careful when running anything found online