Where or how to put transmission oil additive?

Hello,

The transmission check and fill plug is located on the side of the transmission.
In order to check or add any fluid you will have to access the top plug located in the transmission housing.
Correct fluid level is right at or just little below the fill plug.

https://www.2carpros.com/articles/how-to-check-and-add-transmission-fluid

I have attached the fluid checking procedure below.

Please let me know if you have any questions.
Thank you.