It depends on the model of the electric scooter and the seat you choose.
There are two main ways to attach the seat. One way is to screw it into a pre-drilled hole in the scooter deck. The other way is to use a clamping mechanism to hold it in place. Whenever you buy a seat for your scooter, you should make sure it is compatible.
When you want to remove the seat, many models feature a quick-release lever that lets you remove the seat post and saddle, leaving the bottom plate attached so you can easily put it back in place when needed.