I have used all the passwords I know I have (not least the one stored at \home\pgpass) to no avail. I was able to get through just once and since then it has been:
Permission denied, please try again.
What I do is start command prompt as administrator and then
ssh -L 9999:spendwise-XXX.postgres.pythonanywhere-services.com:11942 myusername@ssh.pythonanywhere.com