@_Val_ below an example to create an MTA rule using dbedit commands:
addelement network_objects My_GW mta_settings:domains_rulebase:domain_rules mta_domain_rule ---> will create an empty MTA's rule
modify network_objects My_GW mta_settings:domains_rulebase:domain_rules:0:domain my_domain ---> wil add domain "my_domain" to MTA's rule number "0"
modify network_objects My_GW mta_settings:domains_rulebase:domain_rules:0:next_hop network_objects:my_next_hop ---> will add next_hop "my_next_hop" to MTA's rule number "0"
Hope it will be helpful 🙂