The Server field is used to create hyperlinks to open your page after export from Web Export dialogs and panels.
You can enter the URL to a server in the server field (in the Export Artboard dialog).
Enter your server name and be sure to add a slash at the end if needed. Use the Verify Server button on the right to check that the server is reachable.
If the server is verified a green checkmark icon appears.
If the server can't be found a red icon appears.
The server field does not have to validate to be used.
It is common to forget to put the slash character, "/" at the end of the server value.
After this is setup and after you perform an export you can click on the Server link button and the page will load in your default browser.
You can also open the exported by pages by using Visual Studio Code and Live Server as mentioned in Setting up a Server.