How do I add a bag on Allegiant App?

I am a travel lover and also love to explore different destinations around the world. So, this time I thought to travel to Orlando. I selected Allegiant Airlines and booked flight tickets. After that, I visited their website and got to know about their app. So I was thinking about how do I add a bag on Allegiant App. Moreover, when I visited their website I got to know to add a bag on Allegiant App.