Create Customfield for specific Folder

We need to create Custom Fields over the API. In the API Reference there is the POST Method documented. I wonder now how I can spedify the Folder where the Custom Field should be accessable.

I made a custom Field over the API, this is now existing - if I query the API I get it back. But I can't find it in the Webclient and I can't control in which space is should appear.

Please help!