@Daniele_Sala It looks like @Allen_Alpaca answered your question in your other post. The account number should be your 9 digit account number with a prefix of 1PS (eg if your account number is 123456789 then enter 1PS123456789).
I’m not exactly sure why Robinhood would require a 17 digit number for the delivering account. That could be any random brokerage account and every broker uses different numbering schemes? Maybe that’s just the max?