As Scratch lacks a proper cross-site authentication API, this service unfortunately requires your account credentials.
If you are paranoid about getting hacked, use a different password / username for the account in question, and review the source code.
I'd advise doing this anyways, as some instances of this open source software may be malicious or backdoored.
Otherwise, you are on your own. I am not responsible if your account gets administrative action applied due to abuse of this tool; this is provided for educational purposes.