How is voting for a 3rd party candidate NOT wasting a vote? What 3rd party candidate ever became PRESIDENT?

One time, 160 years ago. The exception that proves the rule.

Sanders is not a Dem. Granted his office is only Senator...but he is 3rd party winner.  Lisa Murkowski of Alaska was defeated in the R primary but won as an write in candidate. 

So it happens just not often. As you said once for the Presidency. This post was edited by my2cents at August 27, 2020 5:55 AM MDT

But those were not presidential elections, to which the question referred. The Electoral College system militates strongly against third-party candidacies.

Our system is geared to the 2 major parties.  Our EC is not biased any candidate has to get to 270. Given that most states are winner take all, the EC does not favor the 2 party system over the many party system. It simply ensures that any winning President must appeal to the entire country not just regional. 
A 3rd party with enough nationwide support can win. This post was edited by my2cents at August 27, 2020 6:06 AM MDT

But the EC also ensures that a candidate must garner enough votes in enough large states, not nationwide, to be elected. This is virtually impossible for a candidate without the endorsement of one of the two major parties.

No....EC requires nationwide support that is its purpose and it is needed.  Without it...CA, NY, TX and FL would be the only states that matter. 

But support in enough states to get enough electoral votes. That's my point.