Place an Area2D node on the powerup. I presume you are using the 2D Editor, so I will be using that as my example. Using the "body entered" function, connect it by doing it in the inspector or use the line:
$Area2D.connect("body_entered", self, "bodyEntered")
Before you play the game, make sure you have made the player in a certain group. If this case, I will be using the group name "Player".
Your function should look something like this (I will be using the code above):
if body.is_in_group() == "Player":
# Do Code