How can i create a pup-up menu like the one for choose the shader on materials in unity 2019?

It took some digging, but I found the shader dropdown implementation in the source reference here. Using that, I put together a quick custom editor that has its own AdvancedDropdown as an example:

 using System;
 using UnityEditor;
 using UnityEditor.IMGUI.Controls;
 using UnityEngine;
 
 [CustomEditor(typeof(ScriptWithDropdown))]
 public class EditorWithDropdown : Editor
 {
     private string selectedItemName = null; // the currently selected item
     private AdvancedDropdownState dropdownState = null; // the stored state of the dropdown
 
     public override void OnInspectorGUI()
     {
         Rect position = EditorGUILayout.GetControlRect();
 
         // set the dropdown label
         var label = new GUIContent("Custom Dropdown");
         if(selectedItemName != null)
         {
             label = new GUIContent(selectedItemName);
         }
 
         // draw the dropdown
         if(EditorGUI.DropdownButton(position, label, FocusType.Keyboard))
         {
             var dropdown = new CustomAdvancedDropdown(
                 dropdownState, Dropdown_ItemSelected);
             dropdown.Show(position);
         }
 
         base.OnInspectorGUI();
     }

     // called from CustomAdvancedDropdown when selection changes
     private void Dropdown_ItemSelected(string item)
     {
         selectedItemName = item;
     }
 
     private class CustomAdvancedDropdown : AdvancedDropdown
     {
         private Action<string> onItemSelected;
 
         public CustomAdvancedDropdown(AdvancedDropdownState state, Action<string> onItemSelected)
             : base(state)
         {
             minimumSize = new Vector2(270, 308);
             this.onItemSelected = onItemSelected; // add item selected callback
         }
 
         // builds the dropdown menu
         protected override AdvancedDropdownItem BuildRoot()
         {
             // create the root element
             AdvancedDropdownItem root = new AdvancedDropdownItem("Root Name");
 
             // add basic child elements
             root.AddChild(new AdvancedDropdownItem("Item 1"));
             root.AddChild(new AdvancedDropdownItem("Item 2"));
             root.AddChild(new AdvancedDropdownItem("Item 3"));
             root.AddChild(new AdvancedDropdownItem("Item 4"));
             root.AddChild(new AdvancedDropdownItem("Item 5"));
             root.AddChild(new AdvancedDropdownItem("etc."));
 
             return root;
         }
 
         // called when an item is selected
         protected override void ItemSelected(AdvancedDropdownItem item)
         {
             // alert item selected
             onItemSelected(item.name);
         }
     }
 }

Hopefully this helps! I can't seem to find any documentation for AdvancedDropdown, but both of the source reference links I provided above give quite a bit of insight.

Good luck!