// POST /admin/newsletters/?opt_in_existing=true
{
    "newsletters": [
        {
            "name": "My newly created newsletter",
            "description": "This is a newsletter description",
            "sender_reply_to": "newsletter",
            "status": "active",
            "subscribe_on_signup": true,
            "show_header_icon": true,
            "show_header_title": true,
            "show_header_name": true,
            "title_font_category": "sans_serif",
            "title_alignment": "center",
            "show_feature_image": true,
            "body_font_category": "sans_serif",
            "show_badge": true
        }
    ]
}

POST /admin/newsletters/
Required fields: name Options: opt_in_existing When opt_in_existing is set to true, existing members with a subscription to one or more active newsletters are also subscribed to this newsletter. The response metadata will include the number of members opted-in. 
// POST /admin/newsletters/?opt_in_existing=true
{
    "newsletters": [
        {
            "name": "My newly created newsletter",
            "description": "This is a newsletter description",
            "sender_reply_to": "newsletter",
            "status": "active",
            "subscribe_on_signup": true,
            "show_header_icon": true,
            "show_header_title": true,
            "show_header_name": true,
            "title_font_category": "sans_serif",
            "title_alignment": "center",
            "show_feature_image": true,
            "body_font_category": "sans_serif",
            "show_badge": true
        }
    ]
}