mailchimp-like free services without the banner, sendinblue and curl

Mailchimp's free plan nowadays has quite an intrusive banner. "Grow your business with Mailchimp". This makes it quite inappropriate for non-commercial emails. Older methods like this one, to make the banner less intrusive, don't seem to work. Looking around for alternatives, one of the top search results was sendinblue, with 300 free emails per day, including transactional emails also. Added bonus - the transactional emails don't even have an unsubscribe link by default, we can add it though. My test implementation is at https://github.com/hn-88/Google-Apps-Script-mailing-list/blob/main/Mailing%20List/Test%20sendinblue%20API.gs

While testing the sendinblue api and the mailchimp api, found some points by trial and error. Just related to curl, syntax of multi-line strings and so on.

1. curl's --user parameter can be translated to the following snippet -
   'headers': {
           "Authorization": "Basic " + Utilities.base64Encode("anystring" + ":" + mailchimpApiKey)
       }
   for use in Google Apps scripts.
   
   
2. curl's --header parameters should be used as such - just take care to escape double-quotes and single quotes inside the strings like \' or \\\' as needed.
   'headers': {
           "accept": "application/json",
           "content-type": "application/json",
           "api-key" :  sibApiKey
       }
3. Sendinblue's transactional emails don't include unsubscribe links by default, but we can add them by including the placeholder {{ unsubscribe }}
   <a href={{ unsubscribe }}>To unsubscribe, click here</a>
   
   
4. Multi-line data needs to be handled with care. In Bash, for curl, we put \ at the end of lines for line breaks between parameters, but for a single parameter (like --data) which has a multi-line parameter, we can use single quotes in Bash+curl and single back-ticks in GAS for json objects as can be seen at the following lines of code here. But for JSON to be valid, there should be no line-breaks within the key-value pairs - so, the htmlContent has to be entered in a single line with no breaks. Even adding \n inside the string causes the API to complain about invalid JSON.