TFS Bug template + Adding custom fields

I am trying to add custom field to a bug in TFS.

I opened the Bug Work Item Type but i have to enter the RefName as show in the image below. Does any bod开发者_如何学Cy know how can create Custom RefName

I tried to do as Ewald Hofman said, i got the below error

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Just enter a name. It is used as the identifier for the column in the rest of the work item and in the warehouse.

A possible format is YourCompanyName.Category.FieldName