how to level’s blueprint function call from widget bp?
You can’t. But you can use an event dispatcher to communicate back with the level BP. Or poll the widget from the level BP.
Something like described in this tutorial here: Unreal Engine 4 Tutorial (BP) - Communicating between widgets and blueprints with Event Dispatchers - YouTube
BTW, this is the VR & AR portion of the forum. You should have posted this in Blueprints.