Workflow interaction with external systems

Hi, where can I find a docu on the following topic?

I want to send a "link" to an workitem to an external (NON SAP) system. A user should be able to click on that "link" and exactely this workitem opens within sap gui.

What opportunities do I have to solve this? In particular I am interested in the call External System => SAP Workflow (item).

Kind regards, thanks a lot in advance.