Sendkeys to ui.WebView
but if there is code that looks for keyevents, search stackoverflow for simulating keyevents in js. If the page has jquery loaded, it is a lot easier.
There are browser security limitations which prohibit directly simulating key events. See Trevor's response above for a complete, if overkill, way to get close
Actually, if you know what the validation code is, you can call that function directly, or even easier, validation code often simply calls another submit function, so you could skip validation altogether and call the submit function directly.
It helps to look at the source code on a desktop pc with a developer console, to figure out the right calls to make with eval_js.
Another option, using eval_js gives you access to the DOM, and you can change the value of form elements, then click submit. Unless the validation depends on key events, this should also work.