I haven’t used this feature, but when reviewing the API, I can see that when you use the createfulfillment() method, it returns an ID.
See: https://www.wix.com/velo/reference/wix-stores-backend/createfulfillment
It seems that you use the sendfulfillmentemail, after you have created/processed the fulfillment first. This first step gives you the fulfillment ID.