set parameters in EventInput in Dialogflow V2 API

This is how I did this:

import dialogflow
from google.protobuf import struct_pb2

session_client = dialogflow.SessionsClient()
session = session_client.session_path(project_id, session_id)

parameters = struct_pb2.Struct()
parameters["given-name"] = 'Jeff'
parameters["last-name"] = 'Bridges'

query_input = {
    'event': {
        "name": "greetPerson",
        "parameters": parameters,
        "language_code": "de"
    }
}

response = session_client.detect_intent(
    session=session,
    query_input=query_input)

Note:
In dialogflow console, you must give default values of parameters as #even_name.parameter_name.
In this case for parameter given-name it would be #greetPerson.given-name and for last-name it would be #greetPerson.last-name.

Docs Reference:
We are using DetectIntent, in which we are using QueryInput, in which finally we are using EvenInput

Hope it helps.