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

Working with sales is a collaborative effort; when you offer them assistance, they're more inclined to involve you in discovery calls. Here are some strategies that have proven effective for me in fostering strong relationships with sales:

Show up in the calls as the product expert: Deepen your knowledge of the product so you can provide valuable insights during prospect interactions. Whether it's troubleshooting, sharing best practices, demonstrating ROI metrics, or sharing use cases from other customers, your expertise adds significant value.

Offer to present or demo: Volunteer to present the product roadmap or conduct a demo during discovery calls. This proactive approach not only showcases the product but also highlights your commitment to supporting sales efforts.

Partner beyond the sale: Instead of viewing interactions as one-off engagements, offer ongoing support and collaboration even after the customer buys the product for example. Offer to have feedback calls, more roadmap sessions, etc.

I would also recommend starting by investing your efforts in a select group of sales reps, build those strong relationships with a few first and then scale over time as needed.