Hey @nejat​! Daniel from Customer Support Engineering here 🔧 

At the moment, it isn't possible to use a custom template when sending an email via API. I know that some of our other users have wanted to see this functionality implemented too, so it would definitely be worth dropping in our Product Wishlist Group to increase visibility for this feature request!

It also is not currently possible to include an unsubscribe link for messages sent via API. This is because they are treated as regular one-off messages that go to directly to a user, as opposed to an email from a routine mass-email campaign!

I know that this isn't the answer you were hoping for, but for now I'd suggest using Outbound emails if you need to include unsubscribe links and/or use custom email templates 🙂