Welcome to the community @kmg!
If the page exists on your site, then just click the Page icon (the one directly to the right of the Link chain icon) and you’ll be able to select from a list of the available pages on the project:
If the page doesn’t exist on the project but is included within the same domain, you can simply enter the page slug including the / prefix (what comes after the TLD) and it will default to the link including the root URL:
Ex: /about/team will link to YourURL.webflow.io/about/team when you’re on a staging site but also YourURL.com/about/team when you’re on the live site URL
If you ever want to link to a page off the root domain, that’s when you’ll need to include the https://YourURL.com portion (as well as the page slug) in the link field.
Hopefully that helps clarify things a bit, but feel free to reach out if you run into any issues 