You can add a button to your website alongside your chat and direct your customers to click on it. Instructions on how to activate it are here:

https://help.upscope.io/en/articles/2801566-let-your-customers-request-a-session-agent-request-button

Alternatively, you can ask your customer to press "Control" or "Ctrl" 5 times to generate a 4-digit code that they will give to the agent to start a session.