MRTK uses what are known as Solvers to allow UI elements to follow the user or other game objects in the scene. The Radial View solver is a tag-along component that keeps a particular portion of a GameObject within the user's view.
You can make a button follow your hand by adding the Radial View (Script) component to the object.
First, drag a button prefab from MixedRealityToolkit.SDK > Features > UX > Interactable > Prefabs to the Hierarchy window.
In the Hierarchy window, select the button prefab. In the Inspector window, click Add Component. Search for Radial View. Once found, select to add the component to the button.
When you add the Radial View (Script) component to the button, the Solver Handler (Script) component is added as well because it is required by the Radial View (Script).
The Solver Handler (Script) component needs to be configured so that the button follows the user's hand. First, change Tracked Target Type to Hand Joint. This will enable you to define which hand joint the button follows.
Next, for the Solver Handler (Script) component, change Tracked Handness to Right. This setting determines which hand is tracked.
There over 20 hand joints available for tracking. Still inside the Solver Handler (Script) component, change Tracked Hand Joint to Wrist so that the button tracks the user's wrist.
Now that the hand tracking is configured, you need to configure the Radial View (Script) component to further define where the button is located and how it is viewed in relation to the user. First, change Reference Direction to Facing World Up. This parameter determines which direction the button faces.
Next, in the Radial View (Script) component, change the Min Distance and Max Distance to 0. The Min and Max Distance parameters determine how far the button should be kept from the user. As a reminder, the unit of measurement in Unity is meters. Therefore, a Min Distance of 1 would push the buttona way to ensure it is never closer than 1 meter to the user.
Now that the button is configured to follow your right wrist, press Play to enter Game mode and test the solver in the in-editor simulator. Press and hold the space bar to bring up the hand. Move the mouse cursor around to move the hand, and click and hold the left mouse button to rotate the hand: