how to get a variable like Layermask but for tags

You need to create a custom Attribute, which is quite complex. However, there is a blog here that talks about this issue. You need to copy the two scripts you TagSelectorAttribute.cs and TagSelectorPropertyDrawer.cs. In particular, the TagSelectorPropertyDrawer.cs script must reside in a folder called "Editor". Then use the [TagSelector] attribute on any string variable.

TagSelectorAttribute.cs

 using UnityEngine;
  
 public class TagSelectorAttribute : PropertyAttribute
 {
     public bool UseDefaultTagFieldDrawer = false;
 }

TagSelectorPropertyDrawer .cs

 using UnityEngine;
 using UnityEditor;
 using System.Collections.Generic;
  
 //Original by DYLAN ENGELMAN http://jupiterlighthousestudio.com/custom-inspectors-unity/
 //Altered by Brecht Lecluyse http://www.brechtos.com

 [CustomPropertyDrawer(typeof(TagSelectorAttribute))]
 public class TagSelectorPropertyDrawer : PropertyDrawer
 {
  
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
     {
         if (property.propertyType == SerializedPropertyType.String)
         {
             EditorGUI.BeginProperty(position, label, property);
  
             var attrib = this.attribute as TagSelectorAttribute;
  
             if (attrib.UseDefaultTagFieldDrawer)
             {
                 property.stringValue = EditorGUI.TagField(position, label, property.stringValue);
             }
             else
             {
                 //generate the taglist + custom tags
                 List<string> tagList = new List<string>();
                 tagList.Add("<NoTag>");
                 tagList.AddRange(UnityEditorInternal.InternalEditorUtility.tags);
                 string propertyString = property.stringValue;
                 int index = -1;
                 if(propertyString =="")
                 {
                     //The tag is empty
                     index = 0; //first index is the special <notag> entry
                 }
                 else
                 {
                     //check if there is an entry that matches the entry and get the index
                     //we skip index 0 as that is a special custom case
                     for (int i = 1; i < tagList.Count; i++)
                     {
                         if (tagList[i] == propertyString)
                         {
                             index = i;
                             break;
                         }
                     }
                 }
                 
                 //Draw the popup box with the current selected index
                 index = EditorGUI.Popup(position, label.text, index, tagList.ToArray());
  
                 //Adjust the actual string value of the property based on the selection
                 if(index==0)
                 {
                     property.stringValue = "";
                 }
                 else if (index >= 1)
                 {
                     property.stringValue = tagList[index];
                 }
                 else
                 {
                     property.stringValue = "";
                 }
             }
  
             EditorGUI.EndProperty();
         }
         else
         {
             EditorGUI.PropertyField(position, property, label);
         }
     }
 }

Source: http://www.brechtos.com/tagselectorattribute/