Whenever I need to draw something, I open https://www.Photopea.com in the browser (loads very fast) and draw the thing. Then I press File - Publish on Imgur (the new panel opens with the PNG image on Imgur). Then I just copy the URL and paste it into a forum / chat.

You can also paste the screenshot into Photopea and draw into it, crop it etc.

BTW. I am an author of Photopea :) It has an API, that allows embedding it into a website (https://www.photopea.com/api/playground), but I am not aware of any large website using it.