To add content to a collection that you have already created with the 'create collection' POST API call, use the /content/add endpoint passing the coluuid and dir (optional) query parameters.
Adding this query paramter will add the file to any collection.
Adding this query parameter will add the file to a specific path in the collection
For more information about this API swagger specification, see here
curl -X POST https://api.estuary.tech/content/add -H "Authorization: Bearer REPLACE_ME_WITH_API_KEY" -H "Accept: application/json" -H "Content-Type: multipart/form-data" -F "file=FILE"