Ok lol, here's one viable method. There's more than one way.
This one will be for searching Tenor.com. Visit the website and search for a description of a gif you'd like. Like this.
Click the one you want, and you'll go to a new page. Right click the gif and select "copy image location" or similar. This is the exact web location of the gif file. Generally we want the file location (URL), not the page. The page URL will sometimes work because the tool is smart. But some websites will be obnoxious and not let you get to the physical file location. It is what it is.
Just come back to the message board post editor you always use and right click -> paste that value. It should turn into a gif right there in the editor. And post!