Installing the knockd daemon, and configuring the sequence and packet types to be used to open and close ports, and using the knock client to actually do it from the other side:
Some plugins may extent the Jenkins API with additional functionality. To [blindly] determine if this is the case for a particular plugin of interest, first enumerate the list of installed plugins:
Then, find the plugin in the list and get the value from its “shortName” field:
That value is the name used in the API URL. In this case, that URL would be
https://<jenkins server>/gerrit-trigger. This might be enough to get you what you need, though, depending on the plugin, you might have to reverse-engineer the plugins’ sourcecode in order to find which further nouns/entity subpaths are available from here. You might do a Google search and then grep for the paths that you find from that in order to discover siblings.
This is not meant to be a complicated post, though the information is made complicated by the effort required to find the information.