The English translation of this sentence is slightly off, gyda'r nos = in the evening, so it should be 'The next time I want a meeting in the evening'

An alternative for 'an evening meeting' would be cyfarfod nosweithiol, nosweithiol being the adjective 'evening'

Since there is not an equivalent adjective for afternoon then no doubt in this sentence 'prynhawn' is behaving as an adjective to describe 'cyfarfod'