How to place a object without tapping on the screen

Although I suggest you place the object when a user taps & where s/he taps on the screen, what you're asking can be achieved like so. (this example is in Kotlin)

Before you begin placing an object, you need to create a ModelRenderable. Declare one @Nullable globally.

private var modelRenderable: ModelRenderable? = null

//Create the football renderable
ModelRenderable.builder()
                //get the context of the ARFragment and pass the name of your .sfb file
                .setSource(fragment.context, Uri.parse("FootBall.sfb"))
                .build()

                //I accepted the CompletableFuture using Async since I created my model on creation of the activity. You could simply use .thenAccept too.
                //Use the returned modelRenderable and save it to a global variable of the same name
                .thenAcceptAsync { modelRenderable -> [email protected] = modelRenderable }

The major chunk of the programming has to be done on the frame's onUpdate method. So you attach a listener for frame updates like so

 fragment.arSceneView.scene.addOnUpdateListener(this@MainActivity) //You can do this anywhere. I do it on activity creation post inflating the fragment

now you handle adding an object on the listener.

 override fun onUpdate(frameTime: FrameTime?) {
    //get the frame from the scene for shorthand
    val frame = fragment.arSceneView.arFrame
    if (frame != null) {
        //get the trackables to ensure planes are detected
        val var3 = frame.getUpdatedTrackables(Plane::class.java).iterator()
        while(var3.hasNext()) {
            val plane = var3.next() as Plane

            //If a plane has been detected & is being tracked by ARCore
            if (plane.trackingState == TrackingState.TRACKING) {

                //Hide the plane discovery helper animation
                fragment.planeDiscoveryController.hide()


                //Get all added anchors to the frame
                val iterableAnchor = frame.updatedAnchors.iterator()

                //place the first object only if no previous anchors were added
                if(!iterableAnchor.hasNext()) {
                    //Perform a hit test at the center of the screen to place an object without tapping
                    val hitTest = frame.hitTest(frame.screenCenter().x, frame.screenCenter().y)

                    //iterate through all hits
                    val hitTestIterator = hitTest.iterator()
                    while(hitTestIterator.hasNext()) {
                        val hitResult = hitTestIterator.next()

                        //Create an anchor at the plane hit
                        val modelAnchor = plane.createAnchor(hitResult.hitPose)

                        //Attach a node to this anchor with the scene as the parent
                        val anchorNode = AnchorNode(modelAnchor)
                        anchorNode.setParent(fragment.arSceneView.scene)

                        //create a new TranformableNode that will carry our object
                        val transformableNode = TransformableNode(fragment.transformationSystem)
                        transformableNode.setParent(anchorNode)
                        transformableNode.renderable = [email protected]

                        //Alter the real world position to ensure object renders on the table top. Not somewhere inside.
                        transformableNode.worldPosition = Vector3(modelAnchor.pose.tx(),
                                modelAnchor.pose.compose(Pose.makeTranslation(0f, 0.05f, 0f)).ty(),
                                modelAnchor.pose.tz())
                    }
                }
            }
        }
    }
}

I used one extension method

//A method to find the screen center. This is used while placing objects in the scene
private fun Frame.screenCenter(): Vector3 {
    val vw = findViewById<View>(android.R.id.content)
    return Vector3(vw.width / 2f, vw.height / 2f, 0f)
}

This is the end result

If you don't want to use hit-testing or button's action methods for placing an object in a real environment, you can use, for instance, a camera's Pose for auto-placement (remember, you have to add an ARAnchor, its corresponding Node and a Renderable in front of the ARCamera i.e. along -Z direction):

if (this.anchorNode == null) {

    Session session = arFragment.getArSceneView().getSession();

    float[] position = { 0, 0, -0.75 };       // 75 cm away from camera
    float[] rotation = { 0, 0, 0, 1 };

    Anchor anchor =  session.createAnchor(new Pose(position, rotation));

    anchorNode = new AnchorNode(anchor);
    anchorNode.setRenderable(yourModelRenderable);
    anchorNode.setParent(arFragment.getArSceneView().getScene());
}

Hope this helps.