// Load envMap.
var textureLoader = new THREE.TextureLoader();
var texture = textureLoader.load('/path/to/env.jpg');

// Load model.
modelLoader.load('/path/to/model.foo', (object) => {
  // Apply envMap to model.
  object.traverse((node) => {
    if (node.isMesh) node.material.envMap = texture;
  });
});