Using a variable in one blueprint that was created in another?

Can I do this with casting? I am also attempting to display it. Here are the details.

I created a login using MySQL Integration that connects to SQL database to verify username and password. I have Set the username in a variable. Once the player successfully logs in another widget opens up which is their player screen. At the Top it says "Welcome ‘username’ " I am then going to use that same variable for another query. Whats the best way to do this?

There are a bunch of ways, including blueprint interfaces, custom events, casting, or just linking to a visible variable in the blueprint. You can do a quick google search to find tutorials on any of these. Also, here is another thread that will talk you through it: Passing variables between blueprints - UE4 AnswerHub