I know this question is a year old, but you can use jquery to do this. If you use the html snippet widget, you can use something like this.
$(".password input").keyup(function(event) {
if (event.keyCode === 13) {
$(".signin").click();
}
});
This requires you to add the class "password" to your password text box, and then the class "signin" to the sign in button.
Hope this helps!
If it's not supported by default, you could perhaps try the Keyboard shortcut widget?
Is there any ‘new’ solution? I tried Mendix Marketplace - Keyboard Shortcut but the Sign-In action is not available (Sign-Out is).