Technically "Conduct" prohibits it, so if a site Mod finds your ad and doesn't like it, it could be deleted.
But advertising is one of the most minor-level TOS offenses I can possibly think of. Indeed there are streams dedicated entirely to this purpose.
imgflip.com/m/ImgflipNews
imgflip.com/m/StreamAnnounce
And again I don't mind it at all on my streams as long as in good taste, etc.