Yeah that seems to be a limitation on how FreePBX lets you manage existing PJSIP extensions. That sucks.
Well you’re going to have to drop your context in [from-internal-custom] so that you can use it. Again, unlike Chan_SIP, PJSIP automatically accepts the messages. It will send them through the same context as calls.
Since you can’t add this as a template for existing extensions, you’ll need to add it to the call context.