Receiving a signal from flask then showing a pop up message when it received a signal

Hello I'm trying to create an app that would receive a signal from my python program and then when it does receive the signal a pop up message would show on my app (I'm using flask btw)

Welcome,

so what have you tried?

Where is your Python program running?

Also, please provide a link to flask for us so we don't have to search through pottery.