how to automatic create a customer field without filling in

[angelia_zhong]

I want to make the customer field (ID) automatic creat if I want to create the issue. 

I put a post function in the transition "create" a sil post-function like that:

number a = random(100000);
customfield_10600 = a;

But why I got the fehler. I really appreicate if someone can help me. Thank you very much.