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
config vpn ipsec phase1-interface
set domain example.com
Changes are effective immediately. After configuring the setting, users will be able to resolve names using single names instead of FQDN.