By Request ID you mean the Identity Request ID created by SailPoint. If Yes you can do so by updating the the plan after Identity Request Initialize step when you would have Identity Request object created and you should have the identityRequestId available in the workflow.
Alternatively you can also use Before Provisioning Rule to update the plan to add the attribute with Request ID. The following is a sample from Before Provisioning Rule which adds request id for Create Operations to descriptions attribute on application.
Once you have the request id in plan you can refer it in SCIM payload using plan variable