Not possible with username and password - only when certificate is used. But:
Previously to iOS 12, certificates that were imported as a .p12 file to the Iphone were visible in Check Point Capsule Connect at the "ENROLL FOR A CERTIFICATE" section. Starting from iOS 12, manually imported certificates are no longer visible in Check Point Capsule Connect and cannot be used.
The only supported method for importing certificates signed by a 3rd party CA to Iphones is by pushing a VPN profile that has the user certificate attached to it. These VPN profiles can be distributed either MDMs or programs such as Apple Configurator.
CCSE CCTE CCSM SMB Specialist