Permissions on new SSRS users with REST API

Hi, 

I'm currently trying to add permissions on new ssrs users with rest api.

Currently, I can create new user but as soon as this user is connected to SSRS, I get an error message rsAccessDenied.

I've searched on the REST API but I haven't found any solution to add permissions for a user to get access to specific report for example.

Every informations about this will be very helpfull!

Thanks.