RSS Feed/News Bad Request on Ajax call

[ENXF NET]

I am searching for username and copying to another field just as example and am getting error 400 Bad Request on ajax call.
Did not find any clear solution on old threads. Can someone help me to solve that?

AJAX:
Code:

function search() {
        var user = $("#user").val();
        $.ajax({
            url: "{{ link('my-page/search') }}",
            method: "POST",
            data: { username: user },
            success: function(data) {
                $("#result").val(data)...

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