Trying to create a "loopback trunk" but can't find "custom dial string"

I am trying to create a loopback trunk so that I can route calls to my DID's back into my PBX.

In this forum people give directions to add a custom dial string of "local/${OUTNUM}@from-trunk"

I can't seem to find the "custom dial string" field on the trunk setup screen.

I am using FreePBX verion 6.12.65-28. Did this field get renamed in this version? If so can someone tell me what the new field is called?