How can I define a variable within App Designer?

Hi, I'm trying to make a simple app in the app designer, however, I can't define variables for the App (see image). Does anyone know why this is the case? And how I could define variables within the app?

Answer 1

[yago urroz castillo]

How do you get to edit this field? I have it locked in my App Designer window.

Answer 2

[Brendan Hall]

I worked it out, I needed to let the app designer know a and b are a thing in properties first, then define what they actually are in the startup function with app.a and app.b before carrying out any functions. (see image)