"If someone claims you're wasting your vote, recognize that they don't care about you. It's a selfish statement. They are saying your beliefs aren't worth being represented, that you should silence your voice so theirs can be louder. Don't give in to that.
Vote your conscience."
- Unattributed quote

After two primary campaigns where the Democratic National Committee used the full force of the Democratic party, the corporate lobby, and corporate media to crush Bernie Sanders' very popular people's movement candidacy. Many Sanders supporters are declaring they're going to write Bernie Sanders in for the general election. 
I'd like to convince them their vote may be more valuable with the Green party.

Green Party.org reports:

"...5% of the popular vote, the Federal Election Commission must classify the Green Party as an official "minor party." This designation will result in approximately $10 million in federal funding for the Green Party's presidential candidate in 2020. "That level of funding," said MGIP Co-Chair Gil Harris, "is a godsend to a party that has been running candidates on a shoestring for decades—doing so partly because our party from top to bottom does not accept corporate money."

Compelling Progressive Reasons to Vote Green

From Hubpages - Green Party member Scott McLarty Gives Compelling Reasons to vote 3rd party:

(1) One should vote according to one's own conscience, ideals, and interests, without surrendering to "experts" who insist that only two parties or two candidates are legitimate.

(2) Al Gore and Hillary Clinton lost for a long list of reasons that have nothing to do with third-party competition, including their own weak campaigns. (In reality, both won the popular vote.)

(3) Democrats assume without evidence that third-party voters would otherwise have voted Democratic, or would have voted at all. Third-party voters have figured out that the Democratic Party isn't going to be rehabilitated or rescued.

(4) If we keep voting for the lesser evil, we're going to be stuck with a dismal choice between the two War & Wall Street Parties forever. The only way to break out of the two-party stranglehold is by voting for other options and building a third-party alternative.

(5) Yes, we need Ranked Choice Voting (RCV). The Green Party has been pushing RCV for decades, while the two-party establishment has been resisting such efforts to make elections fair and open. Democrats say "It's our democracy." There's no such thing as a democracy limited to two parties, which is only a step away from one-party states.

Bernie Sanders is now working with the Biden campaign and the Democrats. I understand the political reality that coerced him to do that. Write-in voting Bernie Sanders, although an effective protest, would be ultimately meaningless. 
I'm not against the idea of protest voting. There's simply a more meaningful use for your vote.
We can empower the Progressive policies Bernie Sanders' brought to the fore , and build a viable, visible alternative to the two-party duopoly. If we're tired of being taken for granted, or completely ignored and dismissed. We need to build our own alternative to Democrats and the two-party duopoly.

“We haven’t had a socialist movement that is a major force in American politics for generations and Bernie’s not helpful with that because he’s bringing them into the Democratic Party,” said Howie Hawkins. “You go into the Democratic Party, they don’t know if you’re a Sanders socialist or you’re a Bloomberg corporatist, right?” - Green Party Presidential candidate Howie Hawkins

See Howie Hawkins interview