KeyCode.Mouse3

Description

Additional (fourth) mouse button.

Use this for detecting mouse button presses. The “3” mouse button is the fourth button on the user’s mouse if any additional buttons exist, for example, extra volume buttons. Unity defines this as the "3" mouse button, as the mouse button numbering begins at 0 for the main mouse button.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Update()
    {
        //Detect if the extra mouse button is pressed
        if (Input.GetKey(KeyCode.Mouse3))
        {
            Debug.Log("Mouse 3 ");
        }
    }
}