How to get template id from shirt id in a script?

You might have to do it manually by finding the image,

Go to a shirt, keep substracting 1 from the ID, if it redirects you to a place, go back one page and then skip that part.

For example,

"312321321" - substract one, you'd get "312321320" if the url still says that it is a "library" or a "catalog" item, you can keep substracting until you get the image id.

if "312321319" is a place, go back to 312321320 and then skip 312321319.

Which means your next link should be 312321318

Keep doing this until you can find the template (image).

if you don't get what I mean, just post a comment.

EDIT: If I comprehensed your question wrong, and you're saying that you already have the shirt, you can just go into properties and find for "ShirtTemplate"

shirt = game.Workspace.Shirt
print(shirt.ShirtTemplate)