How to take the return of a function and throw it into a wappler variable?

[Teodor]

Maybe check the following article it will help you understand how the things work:

Using App Connect with JavaScript Functions

In some cases it's required to read a JavaScript variable or object values and use them in App Connect. In other cases it's required to use a dynamic App Connect expression in your own custom JavaScript functions. Although we don't provide a visual interface for these operations, both are possible with App Connect. We will show you a couple of simple examples, which show you how to do this but it applies for any complex function you might be using. Using App Connect Expressions in JavaScript…

In this specific case your code should become:

function show(n1, n2) {
    var sum = n1 + n2
    dmx.parse('soma.setValue('+sum+')');
}