Changing Features from 'text-davinci-003' to 'gpt-3.5-turbo'

I stumbled upon an open source project about c# integrating with OpenAI API with Httpclient, tested it and implemented some code for null handling, but the bot is for 'text completion'. I kinda wanted the bot to response like from the chatgpt website. Did some numerous code changes but i always get a heapstack of errors. I hope you guys can help 🙂