This is the patter you can follow:
/H/sap_rout/H/your_rout/W/pass_to_app/H/yourapp/S/sapsrv
I will assume the port PI is running on is: 50000 and the SAP router is accessible form the external network without a counter part SAP router.Then you string could look like this:
/H/201.218.215.202/H/192.168.0.1/S/50000
You can find more reference information here: Route Strings - SAProuter - SAP Library