How do I make a tellraw command say someone's name?

I need help here, I am making a map called "The Firing". I want it to say: Welcome to The Firing, [USERNAME]

What I had was

/tellraw @a {"rawtext":[{"text":"Welcome to The Firing, !"]}, {["extra":[{"selector":"@p"}]}]}

It did not work. Please help me here.

tellraw @p {"rawtext":[{"text":"Welcome to the Firing, "},{"selector":"@p"},{"text":"!"}]}