Youtube - ListChannels with Username (forUsername) is not working

I'm trying to get channels info with ChannelsList. This endpoint has a parameter the name is: forUsername but it does not work for this page: https://www.youtube.com/c/FolkartTr

This is my query and it returns empty data:

Somehow, I got this channelID in the page source and its: "channelId":"[UCnS--2e1yzQCm5r4ClrMJBg]".When I try to query with this ID it's okay and returns with correct data.

and this is its payload: http://jsonblob.com/941253671862419456

How will I be able to reach this channel info using a username? I got a few links with usernames and I want to get their info with their username. I don't have any chance to get their channelID's for all.

Thanks for your help.