It is recommended that you use a web browser to post videos to this forum. TapaTalk has limitations on posting videos in that you actually have to hand code the link.
This means you have to act like you are posting an image but instead of selecting the image you select "[CODE]" and then manually enter the code for the video.

This can be a challenge on a mobile device. First you have to get the link you need for the video from the source like YouTube.
In the example below the link is https://youtu.be/3ZAfkkvP8WA
The replace the "[CODE][/CODE]" with similar to the following.

[video=youtube_share;3ZAfkkvP8WA]https://youtu.be/3ZAfkkvP8WA[/video]

For this reason I suggest using the web browser for posting videos.