• Go to Setup > Customization > CRM Fields > Click New.
  • Enter a value on Label field.
  • Choose List/Record on Type field.
  • Choose Transactions on List/Record field.
  • Check on Store Value field.
  • Check on Record is Parent.
  • On Applies to > check on Event.
  • On Display > Parent Subtab assigned your preferred subtab.
  •  On Sourcing & Filtering > Set Filter Using to Type.
  • Set Value is field to Sales Order.
  • Click Save.