How do I instruct my custom GPT to refer to a document or link when giving its answer?

I have built a custom GPT in which I have uploaded 20 documents (the maximum). When a user interacts with the custom GPT, how would I configure it to refer to one of its documents (or an HTTP link if possible) when giving its answer. I tried adding these instructions

Please provide a concise summary or answer based on the information extracted from the referenced document or link. Ensure to include a brief mention or citation of the source material for further reference.

but the GPT does not include any mention of where it got its information in its response.

I tried adding the following policy rules it worked

“Always refer to documents.pdf for any queries related to the product summary”

It has been five months since you posted this. I understand that things may have changed significantly since then. Currently, I am facing an issue where my custom GPT refers to a document I uploaded, but the link does not work and shows “The file was not found.” I am wondering how this process is working for you now. Thanks!