I have recently been investigating using the "Splunk Add-on Builder" to ingest data from a REST API, and I had with a similar issue diagnosing the http requests sent from the created add-on app. The Burp Suite Community Edition app (Burp Proxy manual tool) helped troubleshoot the https request from the created add-on app and the https response from the REST API. The created add-on was configured to use the Burp Suite app as a proxy. The https requests from the created add-on are logged in the Burp Suite app, and the responses from the REST API are logged also. Also, the Burp Suite allows the user to intercept the https requests from the created add-on app, and the user can manually modify before forwarding to the REST API. The Burp Suite Community Edition app is free to download at the https://portswigger.net/burp url. ... View more

It is because Google blocks sign-in attempts from apps which do not use modern security standards (mentioned on their support page). You can however, turn on/off this safety feature by going to the link below: Go to this link and select Turn On https://www.google.com/settings/security/lesssecureapps Moreover google block an ip when you try to send a email since a unusual location, so, you can unblock in the next link https://support.google.com/accounts/answer/6009563 and clicked in accounts.google.com/DisplayUnlockCaptcha . ... View more