The password saved in the DTP UI is encrypted and stored in the PSTRootConfig.xml. If you need to manually override the password for a database connection, you can change the password encrypted property to false and provide a plain text value:
<password encrypted="false">plaintextpassword</password>
The password will continue to be hidden in the browser UI, but it will no longer be encrypted. For security purposes, you should save authentication settings through the database connection page so that the encrypted version of the password is stored.