Arguably, the best things about a parametric modeler are the parameters 🙂 Creating them in Fusion can be pretty clunky though.
The problem: You've spent some effort selecting the right faces, components etc, selected the right tool, gone to enter a value and realized, "This should be a parameter!" Go to the parameter editor window now and you'll have to redo all the work you just did to make your change. There's gotta be a better way!
A solution: In any text field that takes tool input, you should be able to something like:
MyParameterName=34mm
MyParameterName would then be added as parameter and set to 34mm.