What kind of chickens do you have and how high is that opening? Normal adult chickens have no problems jumping up a couple of feet. At 12” high they more step up than jump. Even chicks with a broody just a few days old can normally navigate a jump of 6” or so without really slowing down. They are so busy following Mom that they just go.
I don’t raise the decorative chickens like Silkies that can’t fly or those that have strange things on their heads obstructing vision, so I have no experience with them, but I suspect they would not have much of a problem jumping at least a foot.
My pop door is about a foot off the ground. My adults have no problems with that at all. When I have a broody with chicks I stack pavers to make steps, but after a week they probably don’t need that either. I still leave the steps there for them.