The basic premise of the game was that the owner would have a hidden text link somewhere on the website. On some particularly difficult TCGs the link would actually be hidden inside normal website links, so members would have to highlight over every link to find them.
Finding these links would earn a player a small prize pack and, in the case of older TCGs, a badge.
It's unknown which TCG first created the hidden link, but it is very likely that it was Senshi Card-Mania! This is an assumption based entirely on the fact that Senshi Card-Mania! is the first online TCG, and information on the game can be found on the original website. This is merely a guess and is not 100% guaranteed.
Most modern day TCGs have switched to a find the hidden pixel type games, which is considerably easier.
Some TCGs would award players with a badge to display on their trade posts for finding hidden links.