If you want to use that particular plugin, your best bet would probably to contact the developer 
the API (platform.openai.com) has no access to any custom gpt or plugin stuff found on chat (chat.openai.com)
That said, sentiment analysis isn’t new or hard - there are plenty of repos out there. if you’re talking about this plugin in particular, Sentiment Analysis - New plugin for ChatGPT, the author has a github repo and it looks like they’re fond of the good old ntlk.sentiment kit.