You need to earn some points playing cash games at poker stars. once you complete certain quantity of them, PS will provide you with a surprise chest, open this chests and in some of them you will find those tickets to participate in the weekly tournaments. If you find one, take good care of it because they are really weird to find. Also I think there is a way to buy them but not with money, with stars you also accumulate by playing cash games. I hope this explanation helps you. Good luck
When you play poker, you get points and you get chests, sometimes such tickets fall in chests in red or blue or in others, I sometimes get these tickets but it's rare, in such tournaments you can play on Saturdays if you get such ticket!
The ticket is provided by the chest you earn playing. You can track your chest progress near the Cashier button on the pokerstars software, once you open the chest there are prizes. You can check them on the Pokerstars blog. For the weekly freeroll there are 2different ticket, one is for $5k GTD and another one is for $10k GTD.