Astro took ages to learn this one. I was trying all kinds of different things, and getting kinda frustrated. I found the more he got to practice leaning forward or jumping into a stand, that was what he did, even if he wasn't getting rewarded for it.
The only thing that worked (and I don't know if it'll help you because they're entirely different-shaped and different-thinking dogs), was using my arm as an upright brace, so that he never, ever got to tip forward. I held my forearm vertical, treat at the top, and brought it forward so that it was almost pressed against him. Any time he leapt up, I pulled back, and he couldn't lean forward because my arm was in the way.
He learned to wrap his forelegs around my forearm and nibble at the treat while in the correct position, and I think that helped him build up the abdominal strength to later do it on his own. I faded the "prop arm" to just a luring hand, and then faded that to just a verbal signal. As I faded it I made sure that my hand with the treat was always right over top of him, never too far so that he thought he had to reach for it.
I wish I had video of the process, but nope