Can we use variables?

Is there any way to get at the selected text to use as a variable? I would like to create a wikipedia button that does something like this:

(<a href=\"https://en.wikipedia.org/wiki/Special:Search?search=' + encodeURI($selectedtext) + '\">wiki</a>')

So if I selected the phrase Hello World in my post, it would return this:

Hello World (<a href="https://en.wikipedia.org/wiki/Special:Search?search=hello-world">wiki</a>)