Checkbox

<script type="text/javascript">
function setSubject(oLnk) {
    var aChex = document.getElementById("container").getElementsByTagName("input");
    var aSubj = [], href = oLnk.getAttribute("href");
    for (var i=0,j=aChex.length; i<j; i++) {
        if (aChex[i].type=="checkbox" && aChex[i].checked) aSubj.push(aChex[i].value);
    }
    oLnk.setAttribute("href", href + aSubj.join("-"));
}
</script>

<div id="container">
    <div>Bla 1: <input type="checkbox" value="bla_1"></div>
    <div>Bla 2: <input type="checkbox" value="bla_2"></div>
    <div>Bla 3: <input type="checkbox" value="bla_3"></div>
</div>
<p><a href="mailto:ano@nym.dk?subject=Tilmelding%20til%20nyhedsmail:" onclick="setSubject(this)">Tilmeld</a></p>