Everyone who lives or works in the Netherlands has to have health insurance. In many cases, you will arrange this through your temporary employment agency, which will then deduct an insurance premium from your net salary. You can see this on your payslip.
If your employer does this, your insurance company will send you a confirmation and an insurance card. This will be done within a few weeks after you have registered with them. Ask your temporary employment agency for more information if you do not receive your confirmation and insurance card.