You can do it with this asset for free https://assetstore.unity.com/packages/tools/utilities/transform-reset-31313

You only need to add the following lines above the position button:
if (GUILayout.Button(“Reset PRS”, EditorStyles.miniButton)) {
s.vector3Value = resetScale;
p.vector3Value = resetPosition;
s.vector3Value = resetScale;
serializedObject.ApplyModifiedProperties();
GUI.FocusControl(null);
}