Since your file is in FTP, first you need to download file. You may use WebClient or FTPWebRequest class to download the file. You can load your file to a MemoryStream and let the CSVHelper reads the file from the memorystream. Refer this https://joshclose.github.io/CsvHelper/examples/prerequisites/streams/
A sample is here:
https://github.com/JoshClose/CsvHelper/issues/1412#issuecomment-551122734
Note: for the above sample, it uses MemoryStream, in your case, you can download the file from FTP to byte[] and then construct a MemoryStream from the byte[], then use CSVHelper