The class is not adding

The class is not adding

Hi everybody, i'm trying to add, one of my input a class, if this input is empty, but can't understand why, it does not working
Here is a part of my code:

if(jQuery(this).find(".LinkbuilderForWords").val() == "") {
jQuery("input").find("placeholder").css("background", "red"); 
errors = true;
return;
}