You can navigate to
control -> data section -> Fx and click on
edit to write the Fx formula in a specific editor. Test and save it to see the expected value.
You can refer to the list of supported Fx formulae here.
There is a limitation to the things that you can achieve using Fx functions. e.g. you cant connect to data sources or query your connectors. DronaHQ recommends using JS and Data queries to do advanced data bindings and manipulations.