If you have forgotten your username it can be found in the original welcome e-mail from DEGIRO. If you still have access to your account, your username can be found under Personal Information in the Trader.
If you have forgotten your password, you can use the reset password option on the Login screen by selecting "Forgotten your login details"?
Enter your username and the email address you provided when registering with DEGIRO and choose "Reset Password". An email will then be sent to your registered email address with further instructions.