Escape characters in Set Variable in flow so that I can post to Slack

So, I have a variable, I am carrying from the top - and I have an email, I capture the body from.

Now I want to post that body to slack - of course, I need to escape the HTML characters otherwise JSON format breaks.

How do I use string functions inside this value area? Am I on the wrong track here? 

Thanks to all those who take the time to answer 🙂

Happy new year.