When using VS Code to develop with Object Script, the Server Credentials prompt for connecting to the IRIS instance is happy accept a blank for UnknownUser. See screen shot:
InterSystems IRIS is a Complete Data Platform
InterSystems IRIS gives you everything you need to capture, share, understand, and act upon your organization’s most valuable asset – your data.
As a complete platform, InterSystems IRIS eliminates the need to integrate multiple development technologies. Applications require less code, fewer system resources, and less maintenance.