How to copy custom field value to another project's comment field

[John_Diaz]

When resolving an issue I am trying to copy the values from custom field to append on the comments section of another project.  I tried using a post function "copy value from other field" but I think I need a script to accomplish this.

Here's a more detailed example of what I'm trying to do:

Project A has a custom text field called Justification.  When I resolve an issue in Project A, I want to take the entire contents from Justification and copy it to Project B's Comments field ONLY if an issue from Project A is linked to Project B.