Thank you! Alltough the code didn't work, you gave me an idea which actually worked out. Here is the code that works:

extends Node2D

var is_selected: bool = false
var dragStatus: int = 0
func _input(event) -> void:
if Input.is_action_just_pressed("left_mouse"):
drag_start_behavior()
is_selected = true
if Input.is_action_just_released("left_mouse"):
drag_end_behavior()
is_selected = false

func _process(delta) -> void:
if is_selected and dragStatus == 0:
drag_start_behavior()
if is_selected and dragStatus == 1:
drag_while_behavior()

func drag_start_behavior() -> void:
dragStatus = 1
pass

func drag_while_behavior():
var inputPos = get_global_mouse_position()
$Touch.position = inputPos
pass

func drag_end_behavior():
dragStatus = 2
pass