Add 'template_id' key in X-MSYS-API Custom Header

[Krishnanudhaya]

I had sent a email using SMTP API from spark post and i had a list of template outside the sparkpost. When i send an email, template_id is assigned automatically by spark post. I couldn't override the 'template_id' of email.
Is it possible to template_id using x-msys-api-custom-header?.
Refer : https://developers.sparkpost.com/api/smtp/#header-using-the-x-msys-api-custom-header
Please suggest how to override the template_id of email.