T-SQL trigger that checks if airplane seats are taken

I have a question due to an assignment i am working on at the moment. I need to make a trigger that checks if an airplane seat is taken before a customer can be inserted into the table. I have the following Trigger so far:

CREATE TRIGGER CheckIfSeatIsUnique
ON PassagierVoorVlucht
AFTER insert, update
AS
BEGIN
    IF @@ROWCOUNT = 0 RETURN
    SET NOCOUNT ON

    BEGIN TRY

    IF EXISTS (SELECT 1 FROM PassagierVoorVlucht P Join inserted I on P.vluchtnummer=i.vluchtnummer Where P.stoel = I.stoel)
    BEGIN
        RAISERROR('The chosen seat is taken', 16, 1)
      END
    END TRY
    BEGIN CATCH
        IF @@TRANCOUNT > 0
            ROLLBACK TRANSACTION
        DECLARE @ErrorMessage NVARCHAR(4000) = ERROR_MESSAGE()
        DECLARE @ErrorSeverity INT = ERROR_SEVERITY()
        DECLARE @ErrorState INT = ERROR_STATE()
        RAISERROR (@ErrorMessage, @ErrorSeverity, @ErrorState)
    END CATCH
END

The problem i have, is that the trigger checks if the seat is taken AFTER the insert was done, So the seat will always be taken no matter what.

Is there some way to check if the seat is taken before the insert is done?

thanks in advance