Hi Gareth,
We have a Clarizen function named GetCustomActionUrl('CA name',TargetObject,parameter1,parameter2....)
TargetObject is the request object you want the CA to run on.
Please note the you show transfer the parameters using GetRunTimeParameter() function or GetHiddenRunTimeParameter() (in case you want the values to be hidden from the triggering user.
Please let me know if it worked out for you!
regards,
Tamir