I am writing the paper with my prospective supervisor of the upcoming master's course. Mentioning my upcoming university as an affiliation is not an option as my admission will be finalised in December.
Some journals permit this. Some do not, however. Please confirm with the journal(s) you are interested in, otherwise it might be waste of time for you.
See also https://www.researchgate.net/post/What_can_I_do_when_independent_researcher_does_not_exist_as_an_affiliation_in_the_journals_dropdown_list2 and more discussions on this questions to which I linked in my first reply therein.
If you want to publish a paper before starting your Master’s degree and don’t have an affiliation yet, you can publish as an independent researcher. Make sure to clearly state your situation in the cover letter to the journal. Alternatively, you can collaborate with a researcher who has an affiliation, which can help with the submission process. If you need more guidance, consider reaching out to the journal’s editorial office for advice.