It is important to keep in mind that any software ends up being a living thing. Even if you know all the requirements up front, which you almost never do, they're inevitably going to change down the road. Customers will want new features, business might pivot what they're doing, and so on. It's pretty much inevitable that the software will keep evolving. It's key to recognize this and design things in modular fashion so you can evolve things in a sane way as the need for changes comes up. That said, I find you can start small and figure out what the MVP looks like, then go from there. You can draw out the bare minimum and then use that to interrogate the project manager to make sure it matches what they're expecting.