We recently had a requirement to call Plugin on execute of an Action.
To implement this,
We created a blank Action named Trigger Plugin for custom entity named Security Entity.
Open Plugin Registration Tool and connect to the Org and we will have our action available there as message for our Plugin Step
We can execute our Action from C# and Jscript which in turns will call our Plugin.
Use the SoapLogger tool to get the Soap request
Hope it helps..