Here you have an example of creating an email using outlook interop.
You can set your int to the subject field see below.
using Outlook = Microsoft.Office.Interop.Outlook;
//Create the email with the settings
Outlook.Application outlookApp = new Outlook.Application();
Outlook.MailItem mailItem = (Outlook.MailItem)outlookApp.CreateItem(Outlook.OlItemType.olMailItem);
mailItem.Subject = mailSubject;
mailItem.Attachments.Add(totalPath);
mailItem.Body = mailBody;
mailItem.Importance = Outlook.OlImportance.olImportanceNormal;
try
{
//Try to open outlook, set message if its not possible to open outlook
mailItem.Display(true);
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
return false;
}