What can’t be in an employment agreement?
First of all, an employer cannot ask for a fee to hire you. This is called a “premium” and it is a common problem for migrant workers. This is illegal except under very specific circumstances.
An employment agreement cannot force you to join a union, or prevent you from joining a union.
Any illegal term in an employment agreement has no effect. For example, if the employment agreement says you will receive less than the minimum wage, or reduce your holidays below the legal minimum, you are entitled to the benefits, holidays and so on that the law requires.
If you think there is a problem with your employment agreement, contact me.