How to fit a creda hose?

I have a creda tumble drier. The outlet is on the front door. But the hose keeps falling out. I don't now whether I'm missing a part. I have the hose. And attached to the door is a circular piece of plastic which has a thread in it - about 4 - 5 inches diameter.

I don't know if I should have another pice of plastic that fits into the thread, or whether the hose goes directly into the thread (but keeps falling out).