This is usually quite simple using the Chrome or Firefox developer tools (though I find Firefox a bit trickier).
You can easily spot the license URL because it contains "widevine" or "wdv," (Joyn have drm plus ksm, i don't know why) or because it appears right after the first initial stream URL (e.g.,
https://example.com/stream.mpd, Joyn have
cenc-default.m3u8 or
cenc-default.mpd).
Often, multiple license URLs appear; in such cases, always use the last one.
See here from the Joyn Pro7 Stream:
View attachment 4377419
The license URL consists of a Headers, Payload, and Preview.
You will find everything you need in the Headers for the service PY File.
Scroll down until you see "Request headers" and
:method POST.
And as I said, always use the last license URL (usually the third one), because the initial ones often don't yet contain everything needed for decryption.
The Preview looks like this, so you can be certain that this is the license URL.
View attachment 4377428