Pure agent service in gstr3b

Dear Friends / Experts,

A PCS while rendering his professional service, he also makes MCA fee payment on behalf of Clients and raises bill to Clients by submitting necessary receipts.

Query

Whether PCS should amend for / include ‘PURE AGENT SERVICE’ in GST in addition to his primary service. Can someone clarify on this basic query?

Your reply, please..............

Where do we fill the Bills relating to pure agent service in GSTR3B?

Your reply, please..............

Thanks in advance for possible replies from all concerned.