Add a new layer below labels | NMaps GL JS Docs

Use the second argument of addLayer, you can be more precise when adding a new layer below labels.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Add a new layer below labels</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no" />
<script src="https://cdn.ndrive.com/nmapsgl/v.1.6.8/nmaps-gl.js"></script>
<link href="https://cdn.ndrive.com/nmapsgl/v.1.6.8/nmaps-gl.css" rel="stylesheet" />
<style>
	body { margin: 0; padding: 0; }
	#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<div id="map"></div>
<script>
	// TO MAKE THE MAP APPEAR YOU MUST ADD YOUR ACCESS TOKEN
	nmapsgl.accessToken = '<your access token here>';
	
const map = new nmapsgl.Map({
container: 'map',
style: 'Streets',
center: [-88.13734351262877, 35.137451890638886],
zoom: 4
});
 
map.on('load', () => {
const layers = map.getStyle().layers;
// Find the index of the first symbol layer in the map style
let firstSymbolId;
for (let i = 0; i < layers.length; i++) {
if (layers[i].type === 'symbol') {
firstSymbolId = layers[i].id;
break;
}
}
map.addSource('urban-areas', {
'type': 'geojson',
'data': 'https://d2ad6b4ur7yvpq.cloudfront.net/naturalearth-3.3.0/ne_50m_urban_areas.geojson'
});
map.addLayer(
{
'id': 'urban-areas-fill',
'type': 'fill',
'source': 'urban-areas',
'layout': {},
'paint': {
'fill-color': '#f08',
'fill-opacity': 0.4
}
// This is the important part of this example: the addLayer
// method takes 2 arguments: the layer as an object, and a string
// representing another layer's name. if the other layer
// exists in the stylesheet already, the new layer will be positioned
// right before that layer in the stack, making it possible to put
// 'overlays' anywhere in the layer stack.
// Insert the layer beneath the first symbol layer.
},
firstSymbolId
);
});
</script>
 
</body>
</html>