Since mattermost-plugin version 2.7.1 (where endpoint type was changed from String to Secret) it's not possible to set endpoint configuration for mattermostNotifier with a JobDSL script.

Follwoing JobDS script

mattermostNotifier {
    endpoint('https://MATTERMOST/hooks/xxxxxxxxxxxx')
    notifyFailure(true)
    notifyUnstable(true)
}

causes follwoing error:

ERROR: (unknown source) No signature of method: javaposse.jobdsl.plugin.structs.DescribableContext.endpoint() is applicable for argument types: (java.lang.String) values: [https://MATTERMOST/hooks/xxxxxxxxxxxx]