Well first you need to check if you are colliding with the top of the actor, checking if player y + playerheight/2 < object y - objectheight/2, on keydown of specific key. Then change animation to pick up object, turn off collision, move object up above player, so the player is holding it.
Press key down, change animation to throw animation, move change object xvelocity, yvelocity, and have it constantly degrade over time or until collision with enemy. Change animation of enemy, destroy object.
if you want you player to move while holding the object you have to lock the x and y axis to the player using a pointer reference. Check out this page for details on using pointers to get x and y of another actor and reference it to a different actor :
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