@R_cstalin Try this…  The comments should explain the logic. If you have any questions, let me know. 

Qualtrics.SurveyEngine.addOnload(function()
{
    /*Place your JavaScript here to run when the page loads*/

});


Qualtrics.SurveyEngine.addOnReady(function() {
    var textarea = document.querySelector("textarea");
    var maxLength = 300;

    // Check if listener is already added
    if (!textarea.dataset.listenerAdded) {
        // Create a counter element
        var counter = document.createElement("div");
        counter.id = "charCounter";
        counter.style.marginTop = "10px";
        counter.style.fontSize = "14px";
        counter.style.color = "grey";
        textarea.parentNode.insertBefore(counter, textarea.nextSibling);

        // Function to update the counter and manage text length
        function updateCounter() {
            var currentLength = textarea.value.length;

            if (currentLength > maxLength) {
                textarea.value = textarea.value.substring(0, maxLength);
                currentLength = maxLength;
            }

            counter.textContent = currentLength + " / " + maxLength + " characters";

            // Change counter text color to red if the maximum length is reached
            if (currentLength === maxLength) {
                counter.style.color = "red";
            } else {
                counter.style.color = "grey";
            }
        }

        // Update counter on input and manage text length
        textarea.addEventListener("input", updateCounter);

        // Mark that the listener has been added
        textarea.dataset.listenerAdded = "true";

        // Initial counter update
        updateCounter();
    }
});

Qualtrics.SurveyEngine.addOnUnload(function()
{
    /*Place your JavaScript here to run when the page is unloaded*/

});

In addition to your requirements, I also changed the text color of the “300 / 300 characters” to red once the 300 limit is reached. If you do not want that, just comment this line: 

counter.style.color = "red";

Works perfectly fine for me: 

With maximum characters reached: 

Best
Christian