To file a tax return when you're earning under the threshold, you need to talk to the tax authorities and explain why you want a tax return.
The OP can submit a tax return on a voluntary basis. More details at this link (German):
Here in a few “simple” steps:
Check if paying into pillar 3a makes sense tax wise. This mostly is depends on your commune of residence but also some other factors. You can use the federal tax calculator to get a first estimate, better if you download your cantons tax software and enter all relevant numbers. Stay initially away from pillar 3a health insurance unless you really, really know what it means. Most of those who know will also stay away for ever. Very specially stay away from people which try to sell you a pillar 3a life insurance. They have big financial incentive and will get a commission of many thousand of francs. Which you will pay. A good starter on the topic is this article by The Poor Swiss. Open a pillar 3a account with Frankly/VIAC/Selma/Finpension as they offer the most flexibility and lowest cost. As a fall back you might also open a pillar 3a bank account . Pay as much into it as you can afford, makes sense, but not more than the annual cap. If you are married each spouse can pay up to the limit in their own account. The year after, but before March 31, you request the full tax papers from your cantons tax office. For canton Zurich see link from Mullhollander in the above post. You make a full tax declaration, submit it, and wait for the refund. Hopefully your calculations from step 1 were correct. Otherwise you might have to pay extra tax. For all future years you will get the full tax papers automatically. Tax at source will still be deducted as a pre-payment as long as you are on permit B.