Please use the following code to link a Task item to a Contact. For more details please go thorugh
Outlook Spy. 

Public Const CdoPropSetID4 = "0820060 000000000C000000000000046"

Public Const CdoTask_Contacts = "0x8586" 'Item.Contacts

Public Const CdoTask_ContactsBackLink = "0x8585" 'Item.ContactsBackLink

Public Const CdoTask_LinkedContactsSearchKey = "0x8584" 'Item.LinkedContactsSearchKey


strLContact = "Craig Tadlock" 'Assign Contact Name.

strLContactSearchKey = "12345678910111213" 'Assign proper search key of the Contact.

oFields.Add CdoTask_Contacts, 8, Trim(strLContact), CdoPropSetID4

oFields.Add CdoTask_ContactsBackLink, 8, Trim(strLContact), CdoPropSetID4

oFields.Add CdoTask_LinkedContactsSearchKey, 8, "010000001400000010000000" & strLContactSearchKey, CdoPropSetID4

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