Q.Window boxes
What flowers can I plant in window boxes that will flower and fill it in the shade?

Certified GKH Gardening Expert
There are plenty of flowers that grow well in window boxes and the shade. Begonias, Impatiens, Pansies, and Snapdragons are all compact and shade-loving.
Here's a good list of flowering shade plants:
https://www.gardeningknowhow.com/special/shade/shade-tolerant-flowers.htm
And here's another good list of shade plants that do well in containers or boxes:
https://www.gardeningknowhow.com/special/shade/plants-for-creating-shade-containers.htm