Yes, you can use attribute called placeholder as placeholder=”User name” in input field by editing the file called widget_out.php located in default directory of the plugin but I must suggest that you don’t edit the plugin files, instead you can follow the instructions at page https://www.ads-software.com/extend/plugins/login-with-ajax/other_notes/ under Customizing the Widget and making changes to the file in your theme.
You can hide the existing labels using css display:none
