How to create a pre request body in rest api run collection?

Hi,

we are trying to massively modify the installments invoices with rest api. In the patch we are asked for the installmentsuniqueID which we are trying to recover through a GET runner collection. To launch the GET with runner collections it is necessary to create a pre request body, are there any guidelines on this? It's in some table installmentsuniqueID?

Thanks,

Regards