The donation confirmation that goes to a participant from a Friends Asking Friends event when the event is using Sphere Essentials is system-generated and cannot be edited.  The participant can turn these notifications off in their Participant HQ settings, but no customization to the content can be made.

NOTE: If you need to enter a donation to credit a participant, but don't want the participant to receive this notification you can enter the donation as a General Donation and then go to Bookkeeping > Manage Initiative Records > Search for Donor > Edit to change the credit to the partiicipant. This method will credit them without sending the notification to the participant.