how do I find a lost password in my yahoo messenger account?

There is only one password for Yahoo that gets you in to Yahoo and from there you can enter any site that sill exists. Yahoo messenger no longer exists as it was shut down and closed on July 18, 2018.

You don't because you cannot. Verizon terminated Yahoo Messenger on July 18, 2018 as an outdated app.

Instant Messenger was totally different. Yahoo terminated IM in 2013, more than 6 YEARS ago. Verizon disabled personal email over 2 years ago. 

Messenger is no more for over a year now

Yahoo Messenger has been discontinued. But if you need help resetting a lost or forgotten Yahoo password this Yahoo help article will show you how:

https://help.yahoo.com/kb/account/sln27051.html

Instant messenger no longer exists. It was closed in July 2018: https://help.yahoo.com/kb/SLN28776.html