Navigation.selectOnDown

Description

Specify a Selectable UI GameObject to highlight when the down arrow key is pressed.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;  // Required when Using UI elements.

public class HighlightOnKey : MonoBehaviour
{
    public Button btnSave;
    public Button btnLoad;

    public void Start()
    {
        // get the Navigation data
        Navigation navigation = btnLoad.navigation;

        // switch mode to Explicit to allow for custom assigned behavior
        navigation.mode = Navigation.Mode.Explicit;

        // highlight the Save button if the down arrow key is pressed
        navigation.selectOnDown = btnSave;

        // reassign the struct data to the button
        btnLoad.navigation = navigation;
    }
}