Issue with custom email template

Yes,

you can (perhaps should) change those fields.
Just change it to what you want and select the template in your call to send email. Don't need to put the email template in URL. Do this call:

var emailPayload = {
    template: 'User Registration Default',
    to: {
        name: user.first_name,
        email: user.email
    }
};

That's it. Now the DF will select this template (User Registration Default) and use all config you set. That's why you need to change the default values or create a new template if you'd like to personalize your email messages and settings.