Currently, it is not possible to configure the DNS suffix (search domain) for SSL VPN and IPSEC tunnels through the GUI, but it can be configured using the CLI.

For SSLVPN:

config vpn ssl settings
set dns-suffix example.com example.org
end

For IPSEC:

config vpn ipsec phase1-interface
set domain example.com
end

Changes are effective immediately. After configuring the setting, users will be able to resolve names using single names instead of FQDN.