Kostin, who was drafted by the Blues in the first round (No. 31 overall) at the 2017 NHL Draft, has played in 28 games for the AHL's San Antonio Rampage this season, posting fix goals and six assists.
The World Junior Championship will be held in Vancouver and Victoria, B.C. and begins on Dec. 26. The tournament concludes with the gold medal game on Jan. 5, 2019.
Ten countries will be represented in the tournament: United States, Canada, Russia, Czech Republic, Switzerland, Denmark, Finland, Sweden, Kazakhstan and Slovakia.
