In this case you set in that parameter block and passed that directly to the register call coupled with the other parameters set:
char * ua = "User-Agent: Support";
gc_util_insert_parm_ref(&gcParmBlk,IPSET_SIP_MSGINFO,IPPARM_SIP_HDR,(unsigned char)strlen(ua)+1,ua);
if (gc_ReqService(GCTGT_CCLIB_NETIF,iptbrd,&serviceID,gcParmBlk,NULL,EV_ASYNC) != GC_SUCCESS) {
Jeff