ajax part:

$.ajax({
type:"post",
url:"{:url('Contact/sendEmail')}",
data: "username=" $("#username").val(),
//dataType:'json',
success: function (data) {
alert(data);
}
})

thinkphp part:

//Send email
public function sendEmail(Request $request){
$data = $request->param(); //Return information
$name = $data['username'];

Supplement: There is no problem with the path, and request

is also used to report error:

Not yet Define array index: username