概要

using UnityEditor;
using UnityEditor.IMGUI.Controls;

public class Example : EditorWindow
{
    private SearchField m_searchField;
    private string      m_text;

    [MenuItem( "Tools/Hoge" )]
    private static void Open()
    {
        GetWindow<Example>();
    }

    private void OnGUI()
    {
        m_searchField ??= new SearchField();
        m_text        =   m_searchField.OnToolbarGUI( m_text );
    }
}

SearchField を使用する