The easiest way to send mailers is to add recipients to an existing campaign. Campaigns are associated with a mailing list, so adding a recipient to the corresponding mailing list adds them to the campaign.
1

User Sets Up Campaign in thanks.io dashboard

Details on setting up a campaign can be found here - https://help.thanks.io/create-a-campaign-send-mailers-automatically
2

List All Mailing Lists

List all mailing lists via the API and allow the user to select the appropriate one.See mailing lists for more info.
curl -X GET "https://api.thanks.io/api/v2/mailing-lists/" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY"
3

Add Recipient to Mailing List

Add a recipient to the selected mailing list via the API.See recipients for more info.
curl -X POST "https://api.thanks.io/api/v2/recipients" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "name": "John Doe",
    "address": "123 Main Street, Any Town, NY 12345",
    "mailing_list_id": "MAILING_LIST_ID_FROM_PREVIOUS_STEP"
  }'