Thank you for reaching out to the Community. That is a great question! You are able to add custom fields within your email campaigns that will generate specific information for your contacts. You can click here for more information on custom fields.
If you are wanting to add in your own HTML code, you will need to use our advanced editor for this. Unfortunately we are unable to access the HTML code within the third generation editor templates.
In the meantime, if you have any further questions please feel free to reach out to us here. We are always happy to assist you in any way we can!