No Lawsons is not like ticket master,
it's a convenience store like 7-11.
Looks like this
And they all have machines where you can buy different kinds of tickets for pretty much every event. You can actually see the machine in picture, it's near the window, usually next to a photocopier.
There is also an actual ticket store on Takeshita Street in Harajuku,
Which is the main street you see when you exit the station.