If the problem is that you are trying to upload a video to Overgrow but your Youtube video works:
On your Youtube video page, click on the share button:
Click on the COPY button:
Paste the URL into your post (e.g. the copied https://youtu.be/2K7TU1Hh_3U):
https://youtu.be/2K7TU1Hh_3U
The video will appear as though you’ve uploaded it…