Same just like adding images to description.
.
You can find some gifs from Tenor or any kind of it.

@griges no problem

@MartennLockkenn thank you!

@griges postimages.com enter and copy the section that says direct link

@MartennLockkenn so, same like images, but just gif file?
can you recommend me one, preferably free

youtube?? it can be video?

Usually, you'd have to link to them from an external hosting site like Imgur or YouTube.