Custom attribute that adds a button in front of a variable in the Inspector, pressing which performs an attempt to fill in the object reference with corresponding object that is sitting on the inspected game object.

AutofillAttribute.cs

using UnityEngine;
using UnityEditor;

// REMEMBER that for custom attributes with property drawers, the attribute script should be in normal folder, while the drawer script - in the editor folder

public class AutofillAttribute : PropertyAttribute { }

[CustomPropertyDrawer(typeof(AutofillAttribute))]
public class AutofillAttributeDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        if (GUI.Button(new Rect(position.x, position.y, 50f, 15f), "Fill"))
        {
            string type = property.type.Replace("PPtr<$", "").Replace(">", "");
            property.objectReferenceValue = (Selection.activeObject as GameObject).GetComponent(type);
            if (type == "GameObject") property.objectReferenceValue = (Selection.activeObject as GameObject).gameObject;
        }
        EditorGUI.PropertyField(new Rect(position.x + 51f, position.y, position.width - 51f, position.height), property, label, true);
    }

    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property);
    }
}