KeyCode.Mouse5

Description

Additional (or sixth) mouse button.

Use this for detecting mouse button presses. The “5” mouse button is the sixth button on the user’s mouse if this additional button exists. Unity defines this as the "5" Mouse button, as the mouse Button numbering begins at 0.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Update()
    {
        //Detect if the extra mouse button is pressed
        if (Input.GetKey(KeyCode.Mouse5))
        {
            Debug.Log("Mouse 5 ");
        }
    }
}