Portable AC Port in Apartment Wall
That does not look like a port intended for connecting any sort of hose, particularly as the rim is rounded over without any provision for sealing or securing. It looks like a round register that is missing the central diffuser and damper control parts:

(Photo by Paul Goyette on Wikimedia Commons, CC-BY-SA license)
Do not connect your air conditioner to it as this will most likely disrupt the building's ventilation system (sending your hot exhaust air to other people's apartments) and also probably have flow restriction that will reduce your AC's performance.
Instead, buy an aftermarket sliding door vent kit.