CustomGame.PlayerExists(string battletag, params Region[] regions);
Syntax
public bool PlayerExists(
    string battletag, 
    params Region[] regions
)

battletag
Battletag of user to check exists.
Type: string

regions
Regions to check user.
Type: CustomGameOW.Region

Checks if a player account exists by their battletag. The user must play one full game in an input region to be findable.