How do you plan messaging for a product that isn't fully developed?

This is a tough one to answer without more context on your specific situation, but in general it sounds like you may be being asked to craft messaging for a product you don't believe truly meets customer needs.

When you're in this situation it often means that PMM haven't been involved early enough in the product development process and product are throwing something 'over the wall' for you to launch. This is a common challenge for PMMs, and requires work to get PMM 'a seat at the table' and show how PMM involvement ultimately leads to a better product.

Acknowledging though that this will take time to change and you may be needing to deal with this right now, there are 2 potential routes that could help depending on what your situation is:

1. If it's that your 'must haves' aren't getting through to your product team, dig into why that is, and provide as much evidence as you can as to why the product is not yet meeting customer needs. Doing some testing of generic messaging with potential customers and getting their feedback could help support your case. Try and understand what the pushback is from product, and then look for ways to address their concerns but still get the outcome you want.

2. If there's no potential that the product will be developed further, you could try and find a new angle through customer and market research - is there a niche but underserved market the product could suit? Can you run a beta and try and get some results from customers to use as testimonials and bolster your messaging?