How to create Request Item (RITM) from Service Catalog item using REST API?

We have a service catalog item where people can request resizing of SQL servers, and I have been asked to find a way to raise these via PowerShell & REST API.

I can create incidents and requests, but I can't figure out how to create Request Items (RITMs), which would then automatically create the Request and Task items created when manually filling out the Service Catalog form.

I haven't been able to find any documentation that has been helpful in figuring this out.

Do I need to Post to a specific url? Or should "$ServiceNowInstance/api/now/table/sys_db_object" suffice?
Do I refer to the Catalog Item's sys_id in the request body? If so, what should the format be?

Thanks for any help in advance 🙂