https://stackoverflow.com/questions/29261668/how-to-create-an-ad-hoc-email-and-send-it-using-acumatica

I solved it with the following code:
 

    // Create a new instance of CRActivityMaint to manage activities
    CRActivityMaint activityGraph = PXGraph.CreateInstance<CRActivityMaint>();
    string body = JsonConvert.SerializeObject(jsonObject, Formatting.Indented);

    // Determine the correct RefNoteID based on the entity type
    Guid? refNoteID = null;
    if (entity is SOOrder order)
    {
        refNoteID = order.NoteID;
    }
    else if (entity is SOShipment shipment)
    {
        refNoteID = shipment.NoteID;
    }
    else if (entity is ARRegister payment)
    {
        refNoteID = payment.NoteID;
    }

    if (refNoteID == null)
    {
        throw new ArgumentException("Unsupported entity type. Please pass an SOOrder, Shipment, or Payment.");
    }

    // Create the email activity and populate fields
    CRActivity activity = new CRActivity
    {
        ClassID = CRActivityClass.Email,
        Subject = subject,
        Body = body,
        RefNoteID = refNoteID 
    };

    // Insert and save the email activity
    activityGraph.Activities.Insert(activity);
    activityGraph.Save.Press();
}