Is it Cloud API (platform.uipath.com) or your own instance?
I tested it for myself for the Cloud API on Community Orchestrator and this worked:
{
"startInfo": {
"ReleaseKey": "86c35aee-ef1d-4fa8-a30c-984b91bc6cb1",
"Strategy": "Specific",
"RobotIds": [
168906
],
"Source": "Manual",
"InputArguments": '{"in_inputBox": "some value"}'
}
}
Please try passing this as a header on top of the setting you use in the app:
Also, please beware of the syntax for the input argument.