You create a Custom API message. You need to develop a plug-in for your Custom API. 
Please select all correct statements about this process.

Answer options:

A.You need to know the message name
B.You need to register the plug-in for a step
C.After registration, you need to add the assembly to the solution
D.You need to know the names and types of the API parameters

Correct Answers: A, C and D 
You can extend the Dataverse by creating the custom API that would be exposed as the messages. There are two types of custom messages: Custom Process Action and Custom API. 
The Custom Process Actions messages are the no-code option that you can create using the Workflow Designer. 
The Custom API provides the capabilities of code extending beyond the Workflow. You can use the maker portal, Web APIs, or the Organization service to create the Custom API. The Custom API data is saved in the Dataverse tables. You can invoke the custom message by using the Web API, Organization services, and plug-in.
Option A is correct because you need to know the message name, the names and properties of the message parameters, and the response properties for creating a plug-in for the custom message.
Option C is correct because after the assembly registration, by using the Dataverse Plug-in registration tools, you need to add the assembly to the solution.
Option D is correct because you need to know the message name, the names and properties of the message parameters, and the response properties for creating a plug-in for the custom message.
Option B is incorrect because you don’t need to register the step for the plug-in. You just need to register the plug-in’s assembly and add it to the solution.
For more information about the Dataverse custom messaging, please visit the below URLs: 

https://docs.microsoft.com/en-us/powerapps/developer/data-platform/custom-actions
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/custom-api#write-a-plug-in-for-your-custom-api
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/workflow-custom-actions