Many thanks for your patience. This is the guidance from the Information Commissioner about this:
“The GDPR does not actually specify how to make a valid SAR request. Therefore, an individual can make a subject access request verbally or in writing. It can also be made to any part of your organisation (including by social media) and does not have to be to a specific person or contact point.
A request does not have to include the phrase 'subject access request' or Article 15 of the GDPR, as long as it is clear that the individual is asking for their own personal data.
However, you have a legal responsibility to identify that an individual has made a request to you and handle it accordingly.
Even if you have a form, you should note that a subject access request is valid if it is submitted by any means, so you will still need to comply with any requests you receive in a letter, a standard email or verbally.”
So they cannot force you to fill out their own form as long as it is clear that a request has been made and they would then have to deal with it accordingly.
Does this answer your query?