The first thing you need to use is the on_pressed signal. Its in the ‘Nodes’ tab in the top right. Then you connect that to a script that does something like
func _on_button_pressed():
var rigid_robot_new = load(charcters.tscn).instantiate()
add_child(rigid_robot_new)
OR
var rigid_robot = preload(charcters.tscn)
func _on_button_pressed():
var rigid_robot_new = rigid_robot .instantiate()
add_child(rigid_robot_new)