✅ TL;DR (Too Long; Didn’t Read)
You cannot force or expect institutional shareholders to contact you when selling or pledging stock — but you can position yourself to be on their radar at the right time.
Here’s how:
Monitor 13D/13G, 13F, and Form 4 filings via the SEC or data providers
Build relationships with fund compliance and operations teams, not just portfolio managers
Offer value-added liquidity solutions, not “cold asks”
Work through prime brokers, custodians, or placement agents who already handle those transactions
Use proxy advisory firms or investor relations channels to make yourself visible, not invasive

🧠 Full Answer
Getting institutional holders to notify you before they act on stock movements (especially pledges or large sells) is extremely difficult — and often legally impractical.

But here’s how serious professionals create visibility in that ecosystem:
🔍 1. Track Institutional Filings in Real-Time
Form 13F: Discloses quarterly holdings (delayed)
Form 13D/13G: Discloses active/passive stakes of 5%+
Form 4: Insider transactions, sometimes filed within 2 days

Use tools like: WhaleWisdom, Sentieo, CapEdge, Bloomberg Terminal
These won’t give you advance notice, but they let you build your own alert system to react quickly.

🤝 2. Change Who You Talk To
Most people try to connect with portfolio managers. But large transactions are often executed by:
Operations teams
Compliance/legal
Execution traders
Prime brokers or syndicate desks

Your angle could be:
“I help unlock strategic liquidity for block trades or pledged equity. Here’s how I reduce friction when you need discretion.”
That message gets passed around internally far more than, “Let me know when you sell.”

💼 3. Work with the Right Gatekeepers
Form direct or indirect relationships with:
Custodian banks (State Street, BNY Mellon, etc.)
Prime brokers (Goldman, Morgan, Jefferies, etc.)
Corporate finance intermediaries who help place structured stock-backed loans or pre-arranged trades

Sometimes you need to pay to play: being part of a liquidity desk or working with a FINRA member firm helps.

💬 4. Use IR and Proxy Channels Strategically
Reach out to Investor Relations (IR) teams of the target companies with a clear value offer.
Let them know you’re a potential liquidity provider or strategic buyer and ask to be included in outreach to large holders.
Work through proxy advisory firms or special situations advisors — they often contact shareholders on behalf of companies or bidders.

🛑 What Was Missed or Oversimplified in the Existing Answer
❌ “Just reach out via email or phone” – That’s too vague and naive at this level. Institutional investors don’t respond to cold emails unless there’s a reason and a relationship.

❌ “Investor relations may give you contact info” – Legally, they can’t hand out contact lists for shareholders unless it's public info. You need legal access, not a name from IR.

❌ “Monitor SEC filings” – True, but no guidance on which filings, how to use them, or how to act on the data. Filings are useful for intelligence, not outreach.

🧭 Final Word:
If you want institutions to come to you, you must:
Solve a problem they actually have (liquidity, discretion, execution risk)
Be inside the ecosystem, not just watching from the outside
Offer clear value, and build visibility through credible channels
Good luck and i hope this helps.