How to configure
Go to the Administration page, choose Custom Fields link and add a SIL script custom field.
After you create the field, go to the corresponding Configure link. A page like in the image below will appear:
Click on the Edit SIL Script link to see the SIL editor.
Click on the Save button and the script is associated to the current custom field.
Now type in the SIL script a return value, for example: return priority;
How to use it
Go to an issue and this is how the value of the field is the value returned by the script.
That's it.