do I need to report whether I use subcontractors for the insurance

Subcontractors reported: yes, and the evidence is already in your chain file

Usually you report whether you work with subcontractors. Insurers ask this to properly assess the risk: a subcontractor can bring different liability risks than your own personnel, especially when doing work on your customer's premises. The answer "yes" or "no" is factual and does not change anything in what you already record internally — it is not an assessment of your business operations, only information the insurer needs.

Where it often becomes problematic is what follows: some portals do not just ask whether you use subcontractors, but also whether those subcontractors are themselves insured, whether they fall under your policy, or under their own policy. That is a different answer from the simple yes/no question and requires separate substantiation, for example a policy statement from the subcontractor themselves. If you do not have that readily available, then "no" or "unknown" is the honest answer — do not enter something you cannot support. This question is often linked to does the company have liability insurance for businesses?, because insurers usually place these two questions in the same section of the questionnaire.

No direct legal basis, but a logical place in your chain file

There is no law that requires you to report subcontractors to your insurer; this follows from the terms of your insurer's policy itself, not from legislation such as the Trade Register Act 2007. What is recorded in the Trade Register is the sector in which you are active, and that sector often determines whether subcontractors are a common part of your business operations — relevant to the context of the question, see also in which sector is the company active?. Because the question comes back repeatedly in similar form from different insurers, it is something to record once in your chain file and then reconfirm, rather than having to think through again each time.