Not a simple web-hook, but there are some options that might not require any code on the PD side:

1. Using email to create the ticket:
Under "Applications", click Configure next to Maniphest. There's a section titled "Application Emails", that's designed to create tickets out of inbound emails.
Unfortunately, I don't see any good documentation about it.

2. POST call to Conduit
The Conduit api can be invoked as a POST call using Content-Type: application/x-www-form-urlencoded (basic form post), with each parameter its own field. See cURL at the buttom of https://we.phorge.it/conduit/method/maniphest.createtask/.
If PD supports this form of "webhook", then you should be good.
I think this can also work with query params, but modern methods all require complex json objects as inputs.