Setting values during issue creation based on customer user properties

[systems-intern]

Hello,
when creating issues in the service desk as an agent, the creator must be able to select contracts of the customer. To do this, it must be possible to fill two custom_fields with values automatically. The necessary values are attached to the customer users as user properties. I'm trying to write a script which fills these 2 custom_fields with the user properties customerid and companyid from the "on behalf of" customer in the issue create screen and I can't get anywhere with it. Therefore my question: is this function realizable with Scriptrunner and if yes...how?