I found out that we should use put method instead of post

Also, the API URL must be: api_url = "https://api.cron-job.org/jobs"

Finally, the bare minimum of params should be:

"job": {
"url": command_url,
"enabled": True,
"saveResponses": True,
"schedule": {
    "timezone": "GMT",
    "expiresAt": 0,
    "hours": [hour],    
    "minutes": [minute],
    "mdays": [-1],      
    "months": [-1],     
    "wdays": [-1]       
}

}