<!DOCTYPE html>
<meta charset='utf-8' />
<title>Add Popup to a marker</title>
<meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
<script src='https://api.mapbox.com/mapbox-gl-js/v1.8.0/mapbox-gl.js'></script>
<link href='https://api.mapbox.com/mapbox-gl-js/v1.8.0/mapbox-gl.css' rel='stylesheet' />
            body { margin:0px; padding:0px; }
            #map { position:absolute; top:0px; bottom:0px; width:100%; }
            #markerWithExternalCss {
                background-image: url('marker.png');
                background-size: cover;
                width: 50px;
                height: 50px;
                border-radius: 50%;
                cursor: pointer;

<div id='map'></div>
            //Add your LocationIQ Maps Access Token here (not the API token!)
            locationiqKey = 'pk.a5c3fbf2119bfb2275b62eddbccd76b3';
            //Define the map and configure the map's theme
            var map = new mapboxgl.Map({
                container: 'map',
                attributionControl: false, //need this to show a compact attribution icon (i) instead of the whole text
                style: 'https://tiles.locationiq.com/v3/streets/vector.json?key='+locationiqKey,
                zoom: 12,
                center: [-122.42, 37.779]
            //Two ways to attach the popup to markers
            //1. Add it while creating the marker
            // Create a popup
            var popup = new mapboxgl.Popup()
                .setHTML('<b>Plate Number:</b> SF001<br> <b>Type:</b> Truck');
            var el = document.createElement('div');
            el.id = 'markerWithExternalCss';
            // finally, create the marker
            var markerWithExternalCss = new mapboxgl.Marker(el)
                .setLngLat([-122.464677, 37.777209])
            //2. Add it after the marker is created
            var el = document.createElement('div');
            el.id = 'markerWithExternalCss';
            // finally, create the marker
            var markerWithExternalCss2 = new mapboxgl.Marker(el)
                .setLngLat([-122.421953, 37.764966])
            var popup2 = new mapboxgl.Popup()
                .setHTML('<b>Plate Number:</b> SF002<br> <b>Type:</b> Car');

            //Attach to the marker we just created
            markerWithExternalCss2.setPopup(popup2); // sets a popup on this marker