What should I do if I forget to invoke blessings upon the Prophet (peace be upon him) at the beginning of a supplication?

If one forgets to invoke blessings upon the Prophet (peace be upon him) at the beginning of a supplication (du'a), it is permissible to do so during or at the end of it. This is because it is also one of the occasions for invoking blessings in a supplication. Ibn al-Qayyim mentioned that there are three levels for invoking blessings upon the Prophet during a supplication: First: invoking blessings upon him before the supplication and after praising Allah. Second: invoking blessings upon him at the beginning, middle, and end of the supplication. Third: invoking blessings upon him at the beginning and end of the supplication, with the request placed in between.