GUIUtility.keyboardControl

Description

The controlID of the control that has keyboard focus.

using UnityEngine;

public class Example : MonoBehaviour
{
    // Click on the text field to see the id of the control.

    string str = "A String!";
    void OnGUI()
    {
        str = GUILayout.TextField(str, 10);
        Debug.Log("id: " + GUIUtility.keyboardControl);
    }
}