You can add tags when you create the custom dashboard or when you edit it.
To add tags to a custom dashboard:
Access the Create New Dashboard or Edit Dashboard dialog by creating a new dashboard or editing an existing one. See Creating a custom dashboard or Editing a custom dashboard.
Click Add Tag.
Type a name for the tag or select one from the drop-down list. Tag names can be up to 200 characters long. If you try to specify a tag that is larger than 200 characters, errors will result.
Click Create on the Create New Dashboard dialog or Save on the Edit Dashboard dialog to save the tag with the dashboard.