How do I create value for Sales to encourage them to want to include me in the prospect discovery calls?

Tell them you want to sit in in order to prepare better sales enablement materials for them for future calls, including objection handling documents, pitch decks, feature one-pagers, pricing calculators, and social proof. Tell them you will take detailed notes on what prospects ask for/care about, and tailor future materials to better address those things. This will save Sales lots of talking and provide a better tool box for scaling their own strategy.

I am going to expand the question to be about being included in customer calls in general, not just discovery calls. Personally, I don't think PMMs should be in discovery calls. These calls are often with prospects , with no relationship leverage. If you as a PMM want to learn about what happens in a discovery call, just use a tool like Gong or Chorus

If you don't have access to Gong, and you really want to be on a sales call to learn, here are three framings you can use with your sales counterparts, imagined in how they would introduce you to customers. Note it is generally easier to be on CS led calls with existing customers as there's an existing relationship

Supporting - Perfect when you are supporting a technical discussion, a demo, a roadmap deep dive, or any topic where the rep may not feel super comfortable and can proudly introduce as you as an expert

"Alex is from our product / product marketing team, and he is here to support any questions around our roadmap, or feature xxx etc."

Pitching - Perfect when you are rolling out a new value prop, a new product, a new sales play which hasn't yet seen broad enablement. You could do some sample pitches and show them how its done. Time to walk the talk! Easier with existing customers than prospects

"We've just launched this new product XX which solves problem YY. The product is brand new, and I have invited Alex from our product marketing team to give a bit of an overview "

Learning - You aren't really adding value but could be a fly on the wall. Possible with large group meetings, QBRs, solution presentations etc.

"Here we pride themselves on our customer centricity and Alex from our product marketing team is joining us to learn from these discussions, and make sure we are building the right things and positioning them properly for you.

Even with these framings, if reps are reluctant to invite you -let them be. There are other ways to learn what happens in a customer call, or other ways to talk to customers than impose upon sales teams