Please follow these simple steps:
<!DOCTYPE html>
<html>
<script src="https://aframe.io/releases/1.0.4/aframe.min.js"></script>
<!-- we import arjs version without NFT but with marker + location based support -->
<script src="https://raw.githack.com/AR-js-org/AR.js/master/aframe/build/aframe-ar.js"></script>
<body style="margin : 0px; overflow: hidden;">
<a-scene embedded arjs>
<a-marker preset="hiro">
<a-entity
position="0 0 0"
scale="0.05 0.05 0.05"
gltf-model="https://arjs-cors-proxy.herokuapp.com/https://raw.githack.com/AR-js-org/AR.js/master/aframe/examples/image-tracking/nft/trex/scene.gltf"
></a-entity>
</a-marker>
<a-entity camera></a-entity>
</a-scene>
</body>
</html>