@g-virus said:
I would like to request that you issue me an API key with write access.
Up until now, I have been using a read-only API key to retrieve information about TV shows and movies.
However, I now want to manage my own watchlist registered on TMDB.
I have been using Import List, but as the number of items on my list has increased,
I want to be able to manage it using an API.

If you have an API Key you can already do what you want.
This feature was created when v4 of our API service was developed.
Read the basic instructions here at these links.
v3
https://developers.themoviedb.org/3/getting-started/authentication
v4
https://developers.themoviedb.org/4/getting-started/authorization

My usage was incorrect. Thank you for your response.