Before I report this as a bug and not just related to our Citrix setup, could someone test the following on FileMaker v14 (0v4) on Windows (I know this doesn't happen on a Mac).
In script workplace enter 'Perform Script' and, without clicking in the 'Edit' button, paste the following directly in the 'Optional script parameter' entry below the list of scripts:
//Description of parameter
On our Macs both lines (or as many pasted) are successfully pasted into the script parameter, but on Windows only the first line. This just caught me out passing parameters to a subscript, but as we're on Citrix sessions, can't be sure this happens on all copies of Windows.