Looking for guidance on creating/destroying status window while another function runs

I have a program that opens a window to get input from the user, closes when they click a button, and sends that data as part of a server query for more processing. That query can take a while, especially if there’s a problem (like a timeout) so the user just stares at a blank screen while it’s running. I’d like to have a box pop up with a simple message like “Contacting server, please wait…” and have it close when the query is done. Something like:

var1, var2, var3 = GetVars()
var4, var5 = call_server(var1, var2, var3)

The problem is I can’t find any way in the tkinter library to invoke a window without calling mainloop() and having the program halt while waiting for a button to be pressed or something. I’m starting to look into async (I’ve never used it), but the experimenting I’ve done still stops as soon as it hits mainloop.
Is there anything in tkinter that could do what I’m looking for or do I need to figure out how async works more thoroughly?