I want to scan barcodes with an external barcode scanner

The barcode scanner acts like a keyboard in most cases so most likely all you need to do is set focus on the input field and then scan the value. Then you have something called post amble and that is most likely an enter key. so the form need to have submit on enter. Then you can handle the submit with javascript. So it depends a bit on what kind of barcode scanner you are using.

Take a look at this stackoverflow post on the subject

https://stackoverflow.com/a/21633799/1305117

There is even some javascript for you to use https://stackoverflow.com/a/15354814/1305117

let use this example

<form action="/submitbarcode" method="post">
    <label for="Barcode">barcode</label>
    <input type="text" name="barcode" autofocus>


</form>

<script>
var BarcodeScanerEvents = function() {
     this.initialize.apply(this, arguments);
};

BarcodeScanerEvents.prototype = {
    initialize: function() {
       $(document).on({
          keyup: $.proxy(this._keyup, this)
       });
    },
    _timeoutHandler: 0,
    _inputString: '',
    _keyup: function (e) {
        if (this._timeoutHandler) {
            clearTimeout(this._timeoutHandler);
            this._inputString += String.fromCharCode(e.which);
        } 

        this._timeoutHandler = setTimeout($.proxy(function () {
            if (this._inputString.length <= 3) {
                this._inputString = '';
                return;
            }

            $(document).trigger('onbarcodescaned', this._inputString);

            this._inputString = '';

        }, this), 20);
    }
};
</script>

How do i implement the JS in the form input method?

Seems there is a newer version of the script with "how to use" :)

https://stackoverflow.com/questions/11290898/detect-when-input-box-filled-by-keyboard-and-when-by-barcode-scanner/56540835#56540835

Thanks Alot!