Try evaluating Subscript[a,x].
The keyboard shortcuts won’t work in a browser. I don’t believe there’s a button to do this either.
As a final piece of advice, don’t use Subscripts in code. Always, in every programming language, use descriptive variable names. Traditional math notation is (almost always) not good programming notation.
If you chose to ignore this warning, please see these examples which explain more on the semantics of Subscripts: