In the postcode/zipcode where your site is located.
In the box on the map labelled 'Search for a location' enter your business name and town/city, or alternatively type If you click this you will see the screen below. Above the site name, you will see a Mobile sign in tab. To set the site's location, from the management portal go to Manage > Sites > Select site > Details. It is still possible to allow staff to sign in when they're not on site by using a 'Remote site'.