If are asking about Live tournaments, there isn't a way on the app to do it really. If you are asking about Online/Daily/Correspondence, then you can go to Live, click the + like you are starting a new challenge and choose the Tournaments link.
That will load an embedded browser that will take you to the website. It is probabl possible to also use that to load a Live web interface and sign up to a tourney that way, then back out to the live app interface where it should pair you when the tourney starts, but I don't think that is officially supported.
Can some plz help, can't find the tournament option on the app