func input(event):
if input.is_action_pressed("shield_up"):
$Area2D.monitoring = true
elif input.is_action_just_released("shield_up")
$Area2D.monitoring = false
So pressing input for "shieldup" activates Area, and releasing it deactivates the area
this is not syntax-perfect, but You should get the point. Instead of $Area2D.monitoring You can use $Area2D/CollisionShape.shape_disabled to true or false. There are many more options. Always read about nodes methods in editors built-in documentation
If You still don't get it, You definetely need to dig into some basic tutorials first