To point to the value you want to interpolate, you should first use the tween_property method, passing in the object containing the value and the property name of the value you want to interpolate. Then, in the interpolate_value method, pass in the initial and final values for the property as arguments. For example:
object = { x = 5 }
tween_property(object, "x")
interpolate_value(0, 10)