Inject DB CommandHandler

public class CommandNameHandler : IRequestHandler<source, destination> 
{
     public CommandNameHandler(IDb db)
     {
          _db = db;
     }
      
      private IDb _db;

    // constructors and privates
    public async destination Handle(param, token)
    {
        var submerchantList = _db.SubMerchants.ToList();
        ...
    }
}