How to send Crashlytics log

You are sending the Log properly. But see what Official doc says Logging Caught Exceptions

All logged exceptions will appear as "non-fatal" issues in the Crashlytics dashboard.

To reduce your users' network traffic, Crashlytics batches logged exceptions together and sends them the next time the app launches. If you don't see logged exceptions in your Crashlytics web dashboard, try restarting your app!

In Android, send a custom crash by using

Crashlytics.logException(new RuntimeException("Fake exception"));

Then restart your application

In Crashlytics, select the Non-Fatals

On newer versions of Crashlytics, you need to use these functions:

FirebaseCrashlytics.getInstance().log("Your message goes here")
FirebaseCrashlytics.getInstance().recordException(RuntimeException("Your message goes here"))

Edit:
As @Peter mentioned in the comments, using log alone will not work and you have to use recordException function to bundle your log message along with the exception