1:1 NAT, aka one-to-one NAT or binat, binds a specific internal address (or subnet) to a specific external address (or subnet). Incoming traffic from the Internet to the specified IP will be directed toward the associated internal IP. Outgoing traffic to the Internet from the specified internal IP will originate from the associated external IP.
To allow traffic in from the Internet, a firewall rule must be added on the associated WAN interface allowing the desired traffic, using the destination IP of the internal private IP.
All of the 1:1 NAT mappings are listed under Firewall > NAT, on the 1:1 tab and they are managed from the list on that page.
When adding or editing a 1:1 NAT entry, pick an Interface where the NAT should happen, specify an External subnet IP which is typically a WAN VIP, an Internal IP (or use /32 for a single IP or enter the starting address of the block), and enter a Description. The Destination field should be left set to "any" for most all use cases.
Click Save when finished.