KeyCode.Mouse6

Description

Additional (or seventh) mouse button.

Use this for detecting mouse button presses. The “6” mouse button is the seventh button on the user’s mouse if this additional button exists. Unity defines this as the sixth mouse button because 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.Mouse6))
        {
            Debug.Log("Mouse 6 ");
        }
    }
}