Getting to the bottom of the issue will allow you to prevent the skin discharge from reoccurring, rather than just treating the (annoying) signs of itchiness and discomfort. Your dog will likely need an antibiotic, anti-fungal, or medicated shampoo to relieve the itch and resolve the redness and discharge.
While you’re waiting for your vet appointment, you can help your dog by keeping the affected area clean and dry. You can use COOL water and a cloth to dab the skin, and gently pat it dry.
If you don’t know the cause of the problem, avoid topical creams or ointments until directed by your veterinarian. Putting on that Cone of Shame will also help prevent the problem from spreading.