Good morning

I am hoping that someone would be able to assist with this matter
I am fairly new at using pastel xpress v12 and have a bit of a situation really unsure how to proceed with it.
Our company has been issued a pro forma for an service - we must pay the amount off monthly in increments and they will issue us with receipts and once the service is rendered (in December) they will issue us with a tax invoice.
How do I proceed with this in pastel - as I cannot process this as a normal supplier with tax invoice?

really would appreciate any help

Thanking you all in advance