select.html

<form action="select.php" method="post">
    <select name="redesocial[]" multiple>
        <option value="" disabled selected>Choose option</option>
        <option value="Facebook">Facebook</option>
        <option value="Linkedin">Linkedin</option>
        <option value="Instagram">Instagram</option>

    </select>

    <input type="submit" name="submit">
</form>