How to get a metric CompleteRegistration in the context of campaigns?

Question

I'm trying to get the metric CompleteRegistration in the context of campaigns, but I'm only getting aggregated values by date.

This is my code:

from facebook_business.api import FacebookAdsApi
from facebook_business.adobjects.adaccount import AdAccount
from facebook_business.adobjects.adspixel import AdsPixel
from facebook_business.adobjects.adspixelstats import AdsPixelStats
import json

with open(r"C:\Users\Daniar\facebook_creads.json") as file:
    fb_creds = json.load(file)  

MY_APP_ID = fb_creds['app_id']
MY_APP_SECRET = fb_creds['app_secret']
token = fb_creds['token']

my_app_id = MY_APP_ID
my_app_secret = MY_APP_SECRET
my_access_token = token
FacebookAdsApi.init(my_app_id, my_app_secret, my_access_token)

pixel = AdsPixel(<my_pixel_id>)
pixel.get_stats(params={'aggregation': 'event', 'event': 'CompleteRegistration', 'start_time': '2023-05-16'})

This is part of the output:

[<AdsPixelStatsResult> {
    "aggregation": "event",
    "data": [
        {
            "count": 129,
            "value": "CompleteRegistration"
        }
    ],
    "start_time": "2023-05-16T03:00:00+0300"
}