Btw I need not the string value but the

[blissful wyvern]

So I need to use another approach, not the property drawer?
I need to add a new instance to the list by clicking the button.

[hollow oar]

You can do that through SerializedProperty (and you should)

[hollow oar]

SerializedProperty array = property.FindPropertyRelative("StingsList");

SerializedProperty newElement = array.InsertArrayElementAtIndex(array.arraySize);

newElement.stringValue = "whatever";

And then don't forgot to call ApplyModifiedProperties on the SerializedObject.

[blissful wyvern]

InsertArrayElementAtIndex returns void.
I'm trying like this:

 property.FindPropertyRelative("StatModifiers").InsertArrayElementAtIndex(0);
                DamageModifier elem = property.GetArrayElementAtIndex(0) as DamageModifier;
                elem = new DamageModifier();

but it says annot convert type 'UnityEditor.SerializedProperty' to 'DamageModifier' via a built-in conversion to my "as DamageModifier" conversion.

[hollow oar]

How is DamageModifier serialized? As a normal serialized class/struct or with SerializeReference?

And yeah sorry, I forgot it doesn't return the new element. You'll have to add it and then get it with GetArrayElementAtIndex(array.arraySize - 1)

Or 0 if you want to add it to the start

[blissful wyvern]

With serializeReference like this

[SerializeReference] public List<IStatModifier> StatModifiers = new();


    [Serializable]
    public class DamageModifier : IStatModifier
    {
        [SerializeField] float _damage;

        public void Modify(BehaviourParameters parameters)
        {
            throw new NotImplementedException();
        }
    }

[hollow oar]

You can't cast a SerializedProperty to anything. Everything has to be set through its properties.

For SerializeReference, that would be through the managedReferenceValue property.