You can do a code search without specifying a user/org/repo if you authenticate.
First, generate a personal access token for use for this purpose, from your Profile on GitHub's website:
Settings -> Developer Settings -> Personal Access Token -> Generate New Token (you can leave all access options unticked, since you're just using to make web requests)
Now, your original GET request will work and return results, if you append the token to it:
https://api.github.com/search/code?q=addClass&access_token=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
UPDATE: OCT 2021
As pointed out by a comment below, passing the token in via a query parameter (like above) is deprecated. You must now add it as an Authorization header.
e.g.
curl --location --request GET 'https://api.github.com/search/code?q=addClass +in:file +language:csharp' \
--header 'Authorization: Token xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
or in Python:
import requests
url = "https://api.github.com/search/code?q=addClass +in:file +language:csharp"
headers = {
'Authorization': 'Token xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
}
response = requests.request("GET", url, headers=headers)
print(response.text)