Parameter Description: EmailAdditionalTo sets the value of the Additional To field.
You can specify multiple addresses by separating each with a comma:&EmailAdditionalTo=support@getconga.com,billing@getconga.com.The email address can also be an email address from a Lead, User, or Contact in Salesforce.
You can also use dynamic field content for the parameter value:&EmailAdditionalTo={!Contact.OtherEmailAddress}