In Adiuta, transactions that need to be paid in future are normally done as Bills. This can include expenses incurred, goods purchased, or services provided by a vendor. This way, you can keep track of all your debts, know what or when a bill needs to be paid and how much, for each vendor you are owed.
We recommend to you do bills immediately after getting the service or buy a product on credit, to avoid forgetting and causing data to be inaccurate.
The best time will be when your Vendor or Service Provider issue you an Invoice, but it does not hurt to create the Bill earlier than that if you have all information.