Use javascript click event to set a field value

Whti the html() function I has created a <select> tag and a <button>. When I click the button I get the selected value.

How can I set a table field now with the result?

Here is the code.

<div style="display: flex;align-items: center;">
    <select id="selection2" class="choice">
        <option value="1">Test 1</option>
        <option value="2">Test 2</option>
        <option value="3" selected>Test 3</option>
        <option value="4">Test 4</option>
    </select>

    <div class="component editor button">
        <input type="button" wight="100%" id="btnSelect2" class="nx-button-text blue" style="padding:0 2em" value="Apply" />
    </div>
</div>

<script type="text/javascript">
var button = document.getElementById('btnSelect2');
var selected = document.getElementById('selection2');
button.addEventListener("click", function(event)
{
    event.preventDefault();
    // Table.Field = selected.value; <--- how does this work?
});
</script>