Database
[{"_id":ObjectId("621b6413c07bb9761216d08d"),"name":"test1","created":ISODate("2021-07-12T08:22:16.688Z")},{"_id":ObjectId("621b6413c07bb9761216d08e"),"name":"test2","created":ISODate("2021-08-12T08:22:16.688Z")},{"_id":ObjectId("621b6413c07bb9761216d08f"),"name":"test3","created":ISODate("2021-09-12T08:22:16.688Z")}]
Query
db.collection.find({},{name:1,created:1,externalId:{$concat:["$name","_",{"$dateToString":{"format":"%Y-%m-%dT%H:%M:%S.%L","date":"$created"}}]}})