If you would like to upload a file to a folder that has been shared with you, this can be done using the POST /files endpoint, provided that you have edit permissions on the folder. You will need to provide the folderId you would like to upload to, as well as the path. The path will be used relative to the folderId and will be used as the name of the file upload. For example, a path of "/myNewFile" in the folder with id 1234 would result in a file named "myNewFile" in the folder with id 1234. In order to make use of the folderId you will also need to set calculateFolderPath to false.