Contact.ExternalReference (ext_ref) is not showing in workspace field for contact

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The Contact.ExternalReference (ext_ref)  is not showing in contact workspace. It's a standard filed. I want it to add this field in contact workspace to add some data in it. When I import data using import template, I can add this field in my template, also through rest API, this field is in contact object. But why it isn't shown in workspace fields.