Setting an IP address as a static IP for one of the computers on your network will allow that computer to always use that IP address when available on the network,  you will need to follow these  steps:

Select Network > DHCP Server, DHCP server should already be enabled, as the SonicWall Firewall is the acting DHCP server.

Locate IP address of the computer on the list of DHCP Scope IP addresses

Click on Add Static entry button, check Enable this DHCP Scope

Select the interface which the IP belongs to, you should enter a descriptive label for the static IP in the Entry name field

Enter the IP address in the Static IP Address field, select New MAC address in the Ethernet address field, and enter the MAC address of your computer

Enter Gateway IP address, subnet mask, and click OK.