how to use External REST API for the Transfer price in the SCFO Process.

We are exploring to introduce the external Transfer price REST API which is currently used with certain logic in our on-Prem EBS instance . We are going to use SCFO flow as part of our Intercompany process and going to use the Supply and Transfer Order in the Fusion instance. However we want to derive the Transfer price from our legacy system i.e. EBS and expectations is to use that value during the SCFO process. While we have the External API we need help and guidance for configuration in SCFO process.
The REST api which we have is in EBS system and built using Integrated SOA gateway which is out of box feature of EBS and internally it derive the value from some tables along with our logic which is written in PLSQL.