When I was a kid, I had a paper route. And I had an old single speed bike with the giant chromed rear baskets on it in back. And I could not get my leg over it when there were papers stacked up on the baskets. So, starting as a young kid, I started sharply bending my knee and swinging my knee over my top tube in front of the saddle. Been getting on bikes that way for half a century.
Of my bikes, this one has the highest top tube, which in this case is horizontal instead of sloped, so this bike is the best example for a photo.
Lean the bike towards you to lower the saddle and top tube a few inches, hold your front brake on (or both brakes if you prefer), stand on one leg, then you are a tripod (one leg and two wheels) for stability, and swing your knee over the top tube. Note my posture, I am standing very close to handlebar and leaning over the handlebar. And my knee is above the top tube and in front of the saddle. I had to stand there for several seconds for the camera self timer to show this, if I can do it, you can do it.
If you try it fast, you will fall over. I have tried to expain this to people and they always try it fast, swing their knee into the saddle or for some other reason do something wrong, get frustrated and quit. So, if you decide to try getting on the bike that way, first few times do it very slowly. Perhaps stand on the one leg with the two wheels providing stability for a few seconds with your right foot off the ground so that stable position feels more natural before you sharply bend your knee and slowly (to avoid hitting the saddle) swing your knee over the top tube.
One person that I tried to explain this to could not do it, her knee lacked the flexibility to bend her knee sharp enough. She ended up buying a bike with a woman's frame.
This question comes up often enough that I took this photo to show how I do it two years ago.