If you are trying to use a routed block of public IP address space, then you can go to Organisation/Early Access and enable "NAT Exceptions with Manual Inbound Firewall".
You can technically put a public IP there as long as it is not member of a subnet already on your WAN interfaces. You will however not be able to NAT on your MX towards that public IP. Your internet traffic will not use this interface unless having specific routes via a next hop on that subnet. The rest will just go out the WAN1 or WAN2 interface.
If you mean to use public ip block behind your MX to save on IP's upstream then you will need to put a router in front of your MX that can NAT separately from your MX.
Maybe using Port Forwarding, 1:1 NAT rules, or NAT Exceptions could be a way around it. Just gotta make sure you've got the right network setup and routing config.
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