I have checked out the method within the API and there is nothing that indicates I can post the html content via API.
Is that possible? if so, by which method?
In order to send email content through our API, it needs to be created as a campaign. Once you have a campaign with your content, you can assign it to one or more contact lists and schedule it to be sent. You can read about creating campaigns here: http://developer.constantcontact.com/docs/email-campaigns/email-campaign-api-index.html
While it is not a required field to create a campaign, you will need to assign lists using the "sent_to_contact_lists" property of the campaign. You can find the definition for that here: http://developer.constantcontact.com/docs/email-campaigns/email-campaigns-collection.html?method=POS...
Once you've got your campaign created and lists assigned, the nest step is to scheduled the campaign, and you can find the docs for that here: http://developer.constantcontact.com/docs/campaign-scheduling/campaign-schedule-collection.html?meth...
Lastly, if you have any questions about the docs, please let me know!