How to specify authentication database and target database separately on mongodb connection uri?
Yes.. Easily!

mongo "mongodb://test:testpwd@localhost/test?authSource=admin"
MongoDB shell version v3.4.6
connecting to: mongodb://test:testpwd@localhost/test?authSource=admin
MongoDB server version: 3.4.6
MongoDB&gt; db
test
MongoDB&gt; 


You put your destination DB at end of "base" string and add authentication db to parameter authSource