Prefixes for files on R2

I understand that R2 is object storage and therefore the concept of "folders" is not a natural one for R2. However, I do see a checkboxes in the UI that says "View prefixes as directories". Does anyone know which prefix I can use? I have tried

"folder.myfile.csv" and "folder/myfile.csv" but neither create the logical folder I am looking for. Any ideas?
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