Startjob with inputArgument

I trying to start job with input parameters, but keep failing with

“message”: “Argument Values validation failed.”,
“errorCode”: 2003,
“resourceIds”: null

Documentation only says to add “InputArguments”: “string” in startinfo.

I’m using it like this :
“InputArguments”: “{“username”:username,“env”:env}”
Where i have 2 inputs for my job, username & env.

Can someone enlighten me?
thanks by advance