How to get next SeqNo via projection

Hi 

We are trying to add work location on our employees via projection. By Work Location i mean the work location tab under employee file → Employment → Assignment. 
Via devtools we find that IFS itself uses this call: /main/ifsapplications/projection/v1/PersonnelFileEmployementAssignmentsHandling.svc/CompanyPersonSet(CompanyId='XX',EmpNo='TEST1234')/AssignmentEmployeeWorkLocationSet. 
But when doing this ourselves we have a problem. Because in the request body we are required to send SeqNo. But we dont know how to get this. In the database it is an easy SEQUENCE.NEXTVAL. How do we do something similar to that nextval functionality with projections. 

The only alternative i can see right now is to do a Get call that gets all Employee Work Locations and then find the highest and add +1. But this seems like a bad solution.

We are on 10.8 right now