Example JSON
Note: Where 'tmpl_12345678900987654321' appears, the template ID should be entered.
Where it says 'billingId' you can (optionally) enter an attribute, for example the name of the campaign. You will also see this reference in your overviews and on the invoice.
You determine the merge variables yourself. If you use {{firstname}}
somewhere in the HTML, the variable 'firstname' will be filled in there, which you provide in the JSON in the API call.
Do not use dots in the names of the merge variables.
Also refer to the documentation.