Assign issue to a user that has been selected on a previous screen

[Callum Carlile _Automation Consultants_]

I have a screen with 3 user selection fields, 'Facilities', 'Finance' and 'IT', where a user will be selected for each department.

Later on throughout the process, a sub-task issue is created for each of these departments, linked to their own workflows.

I am looking for a way to automatically assign these sub-tasks to the assignee that is selected in the initial screen. I know that you can use the 'Update issue field' post function to automatically assign the issue to a select user, but am looking for the code that will pull the user selected value from the original form and assign the sub-task to the person that has been selected (I'm assuming I'll have to do this in a custom script runner post function).

I also know this can be done using the JIRA Suite Utilities plugin, however the client that I am doing this for does not want to purchase any add-ons, so ideally I would like this to be achievable using ScriptRunner.

I have searched the internet high and low for the code but can not find it anywhere, please may someone help!