In order to apply the JS condition/action to the FIX message, you should use the Script element.
In order to use the script as a condition/action, it should return the value. In JS every expression returns the value and the result of the script is a value returned by the last run expression. So the boolean value can be returned in such a way:
FIXEdge provides users with the ability to reference parameters defined in the Condition or Action section of the BL rule from inside the script since the FIXEdge 6.14.0 release.
An example of the BL rule:
An example of a reference from inside the script: