Web Link data from Parent record

I'm tried to build web link which take default data for appointment created directly from Service Request;

When I use standard button "Create Appt" Service Request number is populated to new Appointment record;

When I tried to do that via weblink this number is empty(SR number is read only field)

Is there any hidden trick to solve this issue

And what does it mean in links; 

OMCR0=%%%Id%%%&

OMTGT=ApptEditForm&

OMTHD=ActivityNewNav&

OMCBO=Service+Request&

OMRET0=Calendar%3fcal%3d0&

Is there any syntax description how use it; any method how simple create links; any explanation