Transferring multiple data between pages using session storage

I’m making a website that sells keyboards stuff and I want to transfer and display multiple options selected by a user in a text field separated with comas(,). Heres what I have for now.

HTML

<div class="input-name">
                        <label for="products">Cases: </label>
                        <select name="product" id="product" class="larger" onchange="change()">
                            <option selected>--Please select--</option>
                                <script>productlist1()</script>
                        </select>
                    </div>
<div class="input-name">
                        <label for="products">Keycaps: </label>
                        <select name="product" id="product2" class="larger" onchange="change()">
                            <option selected>--Please select--</option>
                                <script>productlist2()</script>
                        </select>
                    </div>

<div class="input-name">
                        <label for="subject" id="subjectLabel">RE: Enquiry on</label>
                        <input type="text" name="subject" id="subject" class="subject_1" readonly>
                    </div>

Javascript

function storeSub() {
    document.getElementById("product").selectedIndex = sessionStorage.productIndex;

    var product = document.getElementById("product").value;

    sessionStorage.subject = product;
    document.getElementById("subject").value = sessionStorage.subject;
}

function productlist1() {
    var select = document.getElementById("product");

    var options = [" Case1", "Case2", "Case3", Case4"];

    for (var i = 0; i < options.length; i++) {
        var opt = options[i];
        var el = document.createElement("option");
        el.textContent = opt;
        el.value = opt;
        select.appendChild(el);
    }
}

function productlist2() {
    var select = document.getElementById("product2");

    var options = ["Keycaps1", "Keycaps2", "Keycaps3", Keycaps4"];

    for (var i = 0; i < options.length; i++) {
        var opt = options[i];
        var el = document.createElement("option");
        el.textContent = opt;
        el.value = opt;
        select.appendChild(el);
    }
}

function change() {
    var product = document.getElementById("product").value;
    sessionStorage.product = product;
    document.getElementById("subject").value = sessionStorage.product; 
}

This is kind of reinventing the wheel. You might consider keeping multiple items in an array, or array of objects, and using JSON.stringify to convert it from an object to string. Later, when you want, read and JSON.parse it back to an array or object.