Hi, as I comment in the post that you reference, you need to get a rect2 box first to make the check. Then, you has to unproject the position of your character from the camera object:
var character_pos = character.get_translation()
var unprojected_pos = cameraObj.unproject_position(character_pos)
With this and your preview rect you can check if is inside of the rectangle.
var rectSelectionBox = Rect2(bla, bla,..,..) #fill with correct values ;)
var contains = rectSelectionBox.has_point(unprojected_pos)
Check the steps in the other post to get the rect2, that was I did and works. All this was made in Godot 2, so , I don't know if will work in Godot3.