First, ensure you have "Auto fill out Forms:" set to ON in iCab settings (press the cog-like icon to get to the settings).
Then, go to a web site where you have to enter your account info. Enter the info but before pressing GO, press the arrow icon in the top bar (on the left-ish side). There should be a line "Save Forms." Press that. Then, either press the login/sign-in button on the web site or bring the keyboard back up to enter GO.
Hope that helps.
Marilyn