Edit in JSFiddle 
<!DOCTYPE html>
<html>
    <head>
        <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://tiles.locationiq.com/v3/libs/maplibre-gl/1.15.2/maplibre-gl.js'></script>
        <link href='https://tiles.locationiq.com/v3/libs/maplibre-gl/1.15.2/maplibre-gl.css' rel='stylesheet' />
        
        <script src='https://tiles.locationiq.com/v3/js/liq-styles-ctrl-libre-gl.js?v=0.1.8'></script>
        <link href='https://tiles.locationiq.com/v3/css/liq-styles-ctrl-libre-gl.css?v=0.1.8' rel="stylesheet" />
        
        <style>
            body { margin:0px; padding:0px; }
            #map { position:absolute; top:0px; bottom:0px; width:100%; }
        </style>
    </head>
    
    <body>        
        <style>
            
            #markerWithExternalCss {
                background-image: url(https://tiles.locationiq.com/static/images/marker50px.png);
                background-size: cover;
                width: 50px;
                height: 50px;
                border-radius: 50%;
                cursor: pointer;
            }

        </style>        
        <div id='map'></div>
        <script>
            
            //Add your LocationIQ Maps Access Token here (not the API token!)
            locationiq.key = 'pk.aa7f5d0539c5675b7f3429402939d8fa';
            //Define the map and configure the map's theme
            var map = new maplibregl.Map({
                container: 'map',
                style: locationiq.getLayer("Streets"),
                zoom: 12,
                center: [-122.42, 37.779]
            });
                        
            //Two ways to attach the popup to markers
            //https://www.mapbox.com/mapbox-gl-js/api#popup
            
            //1. Add it while creating the marker
            // Create a popup
            var popup = new maplibregl.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 maplibregl.Marker(el)
                .setLngLat([-122.464677, 37.777209])
                .setPopup(popup)
                .addTo(map);
        
            //2. Add it after the marker is created
            var el = document.createElement('div');
            el.id = 'markerWithExternalCss';
            // finally, create the marker
            var markerWithExternalCss2 = new maplibregl.Marker(el)
                .setLngLat([-122.421953, 37.764966])
                .addTo(map);
            
            var popup2 = new maplibregl.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

        </script>
    </body>
</html>