Hi @SankarRa1,
Until what I know is that is not possible to access the instance name directly inside a schema field. Schema default values don't have access to context variables like $(login) or values of the instance. If you need to populate a field related to the instance, you will have to set it in a workflow, JavaScript, or a input form where Application.instanceName.
I share with you some useful documentation that can help you:
InstanceNameAPI
Attributes
Schema structure
If you need something more just let me know.
Best,
Celia