Getting in touch
Boxing here is arranged by WhatsApp, directly with the coach — there is rarely a booking system. A short first message that says you are a beginner (or that you have boxed before), asks for the class times and asks for a current rate will usually get an answer the same day. Ask whether an apto físico is needed before your first session, and expect to pay in cash or by local transfer rather than with a foreign card.
The longer version — how this works in Buenos Aires, what to ask and what the Spanish words mean — is on the boxing coaches page.