Sourcing Contract for Subcontracting

Hi Community

I have a question about subcontracting material in contracts.

I have a subcontracting material that not find the contract as a Source in the APP Manage Purchase Requisition. I suppose because we only have the item category standard in the MK / WK contract. How can I still use the contract as source for subcontracting material? Especially that the contract is also recognized as source? What do I have to consider?

Thanks a lot

Best regards Claudia