from https://docs.mapbox.com/mapbox-gl-js/example/set-popup/

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Attach a popup to a marker instance</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no" />
<script src="https://api.mapbox.com/mapbox-gl-js/v1.10.0/mapbox-gl.js"></script>
<link href="https://api.mapbox.com/mapbox-gl-js/v1.10.0/mapbox-gl.css" rel="stylesheet" />
<style>
	body { margin: 0; padding: 0; }
	#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<style>
    #marker {
        background-image: url('https://docs.mapbox.com/mapbox-gl-js/assets/washington-monument.jpg');
        background-size: cover;
        width: 50px;
        height: 50px;
        border-radius: 50%;
        cursor: pointer;
    }

.mapboxgl-popup {
        max-width: 200px;
    }
</style>

// create the popup
    var popup = new mapboxgl.Popup({ offset: 25 }).setText(
        'Construction on the Washington Monument began in 1848.'
    );

// create DOM element for the marker
    var el = document.createElement('div');
    el.id = 'marker';

// create the marker
    new mapboxgl.Marker(el)
        .setLngLat(monument)
        .setPopup(popup) // sets a popup on this marker
        .addTo(map);
</script>

</body>
</html>

https://api.mapbox.com/mapbox-gl-js/v1.10.0/mapbox-gl.js