Trouble adding custom sip header

I’m trying to add a custom sip header to enable a softkey to park calls on grandstream phones. I’ve found that the header should be X-UCM-CallPark: #70 and I’ve found the SipAddHeader function with asterisk, I’m just not sure where to add this to get it to work. I’ve tried adding

exten => s,n,SIPAddHeader(X-UCM-CallPark: #70)

to the extensions_custom.conf file but I’m not seeing the header on a packet capture.