How can I use predefined UI portions as scaffold parameters without getting this warning? > The expression is unused

Try either:

topAppBar = myAppBar

or

topAppBar = { myAppBar() }

Thanks @Brett Best. The problem was I was assigning the definition of a composable to a variable. Instead it should have been a composable to be called when I needed it. As you said, something like

{MyAppBar()}

should be used as the value of

topAppBar

parameter.