cURL > 7.18.0 has an option --data-urlencode which solves this problem. Using this, I can simply send a POST request as
curl -d name=john --data-urlencode passwd=@31&3*J https://www.example.com
Summarizing the comments, in case of mixed “good” and “bad” data and exclamation marks inside we can use on Windows:
curl -d "grant_type=client_credentials&client_id=super-client&acr_values=tenant:TNT123" --data-urlencode "client_secret=XxYyZ21D8E&%fhB6kq^mXQDovSZ%Q*!ipINme" https://login.example.com/connect/token