Example.cs

//Example
public void AddFriend()
{
    //Show Input Window
    if (inputWindow != null)
    {
        inputWindow.onOkClickCall += AddFriend2;
        inputWindow.ShowInputWindow("Username to add as friend:");
    }
}

public void AddFriend2()
{
    //Do something with inputWindow.result
}

InputWindow.cs

//Made by: HolyFot
//License: CC0 - https://creativecommons.org/share-your-work/public-domain/cc0/
//Easy re-usable Input menu using a delegate call back.
using System;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using UnityEngine.Events;

public class InputWindow : MonoBehaviour
{
    public Text msg;
    public GameObject window;
    public Button OkBtn;
    public Button CancelBtn;
    public InputField input1;
    public string result;

    public delegate void EventInputEntered();
    public event EventInputEntered onOkClickCall;

    void Awake()
    {
        if (CancelBtn != null)
            CancelBtn.onClick.AddListener(delegate { HideWindow(); });
        if (OkBtn != null)
            OkBtn.onClick.AddListener(delegate { OkClick(); });

        HideWindow();
    }

    public void ShowInputWindow(string message)
    {
        msg.text = message;

        window.SetActive(true);

        //Set Focus
        input1.ActivateInputField();
        input1.Select();
    }

    public void OkClick()
    {
        result = input1.text;

        if (onOkClickCall != null)
        {
            onOkClickCall.Invoke();
        }

        HideWindow();
    }

    public void HideWindow()
    {
        window.SetActive(false);
    }
}