As stated in IAB’s implementation document, the SupplyChain object should be included in the BidRequest.Source.ext.schain attribute in OpenRTB 2.5 or later. For OpenRTB 2.4 or prior, the BidRequest.ext.schain attribute should be used. 

Additionally, SupplyChain information can be communicated via a tag rather than OpenRTB. This situation most commonly occurs when an advertising system provides a tag that can be inserted into an ad server, video player, SSAI vendor, etc. to initiate an ad request to an advertising system. 

You can read more about the specs and proposed implementation in the following link:        

https://github.com/InteractiveAdvertisingBureau/openrtb/blob/master/supplychainobject.md