Doesn't watch the source nor reads the provided info in the video notes itself, wants proof for something that is derived from the original sources. Can't even fatom what you wanted to achieve with this comment.
The provided video description and pinned comment where added after my comment. Please be more careful with your sarcasm.
As for the Wikipedia source, if you read the full article (it's not too big, give it a try) it can be interpreted differently from "fighting for the same cause as the group" compared with "volunteering to join forces with the group" when put side by side with the OP's statement that I asked sources for. One thing is to do a one off action in association with a specific group, another is to fully volunteer towards the group. Hence why I wanted more sources to disambiguate the word "volunteer".
https://www.lepoint.fr/societe/ces-jeunes-partis-combattre-l...
https://www.mediapart.fr/journal/international/010919/ces-re...