It’s great to hear that OSM and uMap are useful to you and that you want to give back.
See donate.openstreetmap.org for opportunities to support OpenStreetMap as a whole. A company can also support the project by becoming a corporate member. Both of these go through the international OpenStreetMap Foundation.
However, the popular uMap site you’re linking to is hosted by the French local chapter as far as I know. So maybe you want to support them directly as a thank you for providing that service: https://www.openstreetmap.fr/association/
I’m not sure if the developers of the uMap software also welcome donations, maybe they can chime in if that is an option.
Thanks very much @Tordanik, I will try to contact the French local chapter and if I can’t get through I will donate via the standard route or as a corporate member.