I don't have access to the Azure Portal currently to confirm, but:
1. Log in to Azure
2. Go to Monitor and click Virtual Machines (under Insights)
3. Can you change the Workspace configuration here?
If you can't (it is a good opportunity for a Feature request), you might have to do this with PowerShell, take a look at my blog post for a script that will get you 80% of the way there - https://luke.geek.nz/powershell/add-log-analytics-workspace-to-microsoft-monitoring-agent-via-powers...
You might just have to wrap a ForEach around it to connect to each server and add it.