As per our documentation creating trigger will only be possible like below
Triggers can be created for services, roles, role configuration groups, or hosts. Create a trigger by doing one of the following:
Directly editing the configuration for the service, role (or role configuration group), or host configuration.
ClickingCreate Triggeron the drop-down menu for most charts. Note that the Create Trigger command is not available on the drop-down menu for charts where no context (role, service, and so on) is defined, such as on theHome>Statustab.
Use theCreate Triggerexpression builder. See documentation link below