Whoever has not yet applied for a healthcare allowance can still do that via the tax service. You can do that online (in Dutch) via www.toeslagen.nl
or by telephone (belastingtelefoon): 0800 - 0543.
Either on the form they will send you or online you will need to fill in what you expect to earn in the coming year. The tax service will determine the amount of the allowance based on that information. If you live alone, you can receive up to €35 a month. If you have a partner, you can receive up to €100 per month (per household).