Let me see if I can get this to work in the wysiwyg editor:
This is a test
Which definitely works. I think that the issue you are having is that you are trying to use the fas class, which I'm not sure is available in Font Awesome 4.7 (the version we are currently using in Crowdstack). We have plans to upgrade this in an upcoming release.
Next, since the WYSIWYG does not have Font Awesome icon support built directly into it you will need to use the "Source code" option on the far right to edit the posts HTML and manually insert the icon using code like this:
<span class="fa fa-power-off"> </span>
That is what I used above, and it appears to work just fine.
After Edit Notes:
Okay, thanks to the WYSIWYG massaging of the HTML this actually did end up requiring a bit more work to massage it into something the editor would not mangle. First, the element has to have something inside of it for the WYSIWYG to not strip it out and replace it with double BRs. Second, I needed to include other words either before or after it in order for it to not massage the classes into the paragraph and drop the span. Third, I had to use a span instead of a "i" since the WYSIWYG would not only strip the i out if it was the only HTML, but would apply a "style='font-style: italic;'' attribute to the paragraph.
Bottom line, it seems you are better off using image icons in your posts instead of trying to rely on font icons at this time. It can be done, as I showed above, but because of how the WYSIWYG likes to massage the HTML when it's posted it can be a tiresome process requiring a lot of iterations to get it to look right.
Hope this helps!