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