My understanding is that for workspaces and portals  'normal' URL navigation is not available and you need to identify the target record via sys_id . So if you need to use workspace (and no one has a better answer), then you will need to update your integration\syncing with your external platform and pass sys_id as a parameter.
If you just need access to the case (and have access rights) you should be able to navigate to it directly eg
https://<instance_name>.service-now.com/sn_customerservice_case.do?sysparm_query=number=<case_number...

Examples of navigating by URL (servicenow.com)