Unfinished prerequisite undergrad course in the last year in MS application as a final year undergrad student
Usually transcripts include registered courses as well as graded ones, and usually you'd be registered for those courses already when you apply. It's normal for anyone applying to graduate school as an undergrad has half of their senior yet to still complete. It's a totally normal situation completely familiar to admissions committees.
If for some reason your transcripts don't show future enrollment, sure, it's fine to mention somewhere, but no need to make a big deal about it (just a list of enrolled courses for spring would be sufficient). Your acceptance will be conditional on completing your degree, including any required courses you still have in progress.