How do i make system field drop values dependant from custom fields selected value

[Margarit Holm]

I have system field AffectsVersions  with options a,b,c and custom field A with options a, b, c. They all have multiple values to select. So what i need: if i choose option a from custom field a, i want the option for AffectVersions to be a as well, without me choosing it manually. I found a javascript code, but unfortunately its not working in Jiras script-runner. are there other possibilities? Her is the js code:

<script type=“text/javascript”>
source = document.getElementById(‘customfield_10025’);
if (source) {
target = document.getElementById(‘AffectsVersions’);
source.onchange=function() {
if (this.value == a) {
target.value=a;
}
}
}
</script>