params is for GET-style URL parameters, data is for POST-style body information. It is perfectly legal to provide both types of information in a request, and your request does so too, but you encoded the URL parameters into the URL already.

Your raw post contains JSON data though. requests can handle JSON encoding for you, and it'll set the correct Content-Type header too; all you need to do is pass in the Python object to be encoded as JSON into the json keyword argument.

You could split out the URL parameters as well:

params = {'sessionKey': '9ebbd0b25760557393a43064a92bae539d962103', 'format': 'xml', 'platformId': 1}

then post your data with:

import requests

url = 'http://192.168.3.45:8080/api/v2/event/log'

data = {"eventType": "AAS_PORTAL_START", "data": {"uid": "hfe3hf45huf33545", "aid": "1", "vid": "1"}}
params = {'sessionKey': '9ebbd0b25760557393a43064a92bae539d962103', 'format': 'xml', 'platformId': 1}

requests.post(url, params=params, json=data)

The json keyword is new in requests version 2.4.2; if you still have to use an older version, encode the JSON manually using the json module and post the encoded result as the data key; you will have to explicitly set the Content-Type header in that case:

import requests
import json

headers = {'content-type': 'application/json'}
url = 'http://192.168.3.45:8080/api/v2/event/log'

data = {"eventType": "AAS_PORTAL_START", "data": {"uid": "hfe3hf45huf33545", "aid": "1", "vid": "1"}}
params = {'sessionKey': '9ebbd0b25760557393a43064a92bae539d962103', 'format': 'xml', 'platformId': 1}

requests.post(url, params=params, data=json.dumps(data), headers=headers)