FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 190760
This article describes how to configure a third party DDNS service.
This scenario assumes that  a DDNS service contract is already active and all the parameters from the third party DDNS service are available.

Execute the following commands from CLI.
# config system ddns
    edit 1
        set monitor-interface "wan1"
        set ddns-server ddnsservice.example
        set ddns-domain "hostname"
        set ddns-username "username"
        set ddns-password password

Add the below vendors DDNS on the FortiGate.                       <----- and                         <-----                          <-----                        <----- Peanut Hull.                       <-----                       <-----                          <-----                               <-----
genericDDNS                      <----- Generic DDNS based on RFC2136.
FortiGuardDDNS                   <----- FortiGuard DDNS service.                         <-----
'wan1' is the WAN interface bound to the DDNS service.
'' is the name of the DDNS server from the DDNS service provider.
'hostname' is the name assigned to the FortiGate.
'username' is the user account for the DDNS service provided by the DDNS service provider.
'password'is the password of the account for the DDNS service provided by the DDNS service provider.

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Technical Note: How to configure a third party DDNS service