How can I make a button with the "+" plus symbol to add to a EditorGUILayout.Popup ?

This is my code inside the OnGUI

Code (csharp):

1.  
2. void OnGUI()
3.     {
4.         int selected = 0;
5.         string[] options = new string[]
6.         {
7.      "Option1", "Option2", "Option3",
8.         };
9.         selected = EditorGUILayout.Popup("Label", selected, options);
10.      }
11.  

The result is :
The label is in black on the left and the popup with the options 1,2,3 in more or less in the middle :



And this is how I want it to looks like :



I'm not sure how to create this custom popup with the "+" plus on the right.