Creating a Virtual Assistant for Client Google Account Integration

Hi! I'm trying to create a Virtual Assistant agent that I can provide to a client such that it can manage their contacts, calendar events, todo, etc. To keep things simple I'm just using Google Sheets and Google Calendar. My client will interact with the VA agent via slack. In development, I've connected the agent to my own google account, however when I deploy the agent to my client, it will, of course, need to connect to their google account. I can't seem to find how to do this. In my mind the best experience would be: My client starts using the agent, and when access to google sheets, etc. is needed, they are presented with "sign in with google" button to connect their accounts. Any help with this would be greatly appreciated!