How to get attribute value of other work item using Calculated value scripts
Accepted answer
For all I know this is not supported in attribute customization. See https://rsjazz.wordpress.com/2016/07/15/rtc-process-customization-what-you-can-and-cannot-do/. With attribute customization I mean what is covered in https://jazz.net/wiki/bin/view/Main/AttributeCustomization and https://jazz.net/library/article/1093 .
There is a work item proxy which is not supported API that has been used for access across links https://www.google.com/search?q=wiork+item+proxy+site%3Ajazz.net. It might be possible to use REST and other calls as well.