Hoe to setup this internet connection ?

I have a test firewall in office and this network:

                 Internet
                       |
     modem/router (with dhcp)
                       |
                   switch
                       |
           [Network        192.168.12.x ]
                       |
           [My test zone 192.168.12.x]
                       |
                    (EXT int 192.168.12.15)
            [Firewall]
                    (INT int 192.168.1.15)
      
The question: how can i set internet connection on the firewall ? Do i need to add it like an interface ?

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