Using following JS script code user will register to provided segment, at the time of allowing notification permission.
Subscribing for Single Segment
<script>
var _pa = [];
_pa.push('YourCreatedSegmentName');
</script>
Subscribing for Multiple Segments
<script>
var _pa = [];
_pa.push('YourCreatedSegmentName1', 'YourCreatedSegmentName2');
</script>