You've built an HTML webform for new members to subscribe to your list that uses CURL to call your subscription URL. When a new user is created using this method their mail format is set to Text and you would like their mail format to default to Multipart-Alternative.
In order to control the mail format for a subscriber, include
&mailformat_=<FORMAT> in your URL according to the solution below where
<FORMAT> is T for Text Only, M for Multipart-Alternative, or H for HTML.
- Construct your subscription URL using this example to set users to Multipart-Alternative:
Change the 'M' in the example above to 'T' for Text Only or 'H' for HTML. New users who use your webform constructed in this manner will be subscribed with the mail format that corresponds to your setting of 'M', 'T', or 'H' in the URL.