For Platinum account and above, you can add username and password when editing/creating cronjob at Send HTTP Request > Username and Password.
For all account, you can just add username and password directly to cronjob URL like this
Replace username and password with your real values.
Username should contain only letters, numbers, dashes (-) and underscore (_). If you want to add other characters, just let me know.
Password should be URL encoded. You can use this tool to encode it, e.g. it'll convert from hello*world! to hello%2Aworld%21