Scriptlistener form submitted

Hello, I want a script listener to run when a form is submitted (rather than an issue is created). However I can find that type in the 'On these events'.

I therefore tried to run a post function on a workflow transition which would give me the same trigger. In the condition field I need to condition it further. However for a post function it is not as easy to make a condition like it is in a Script listener as you don't have the expression builder and that type of language. 

If I make the condition in the expression builder and then paste it to a post function it isn't accepted. 

Therefore I guess I have two questions:

1. Is there a way to set up a 'on these events' in a script listener that is the equivalent of a 'form submitted' so I don't have to use a post function

2. If not how do I write the below expression that will be accepted in the post function expression? 

issue.customfield_14407 ?