There is no web service which can be called to directly create sourcing request in Ariba.
There are two ways which helps users to create sourcing request.
1. Login to Ariba -> Create -> Sourcing request
2. If Ariba system is integrated with SAP ECC system and RFQ is created in ECC and routed to Ariba - In this care as soon as RFQ document reaches to Ariba in cXML format, sourcing request is automatically created in Ariba system.(
Please refer below screenshot for detail.