Pre-approval requires an analysis of your finances, which includes a credit check. These credit checks are what are known as soft credit enquiries. These do not impact on your credit score. However, it’s best to focus on improving your credit score before applying for pre-approval.
You can achieve this through such measures as paying off debt, paying your accounts in full and on time, and keeping credit card debt low. That way you are far more likely to have a fair, or good to excellent credit score when you apply for pre-approval, improving your chances of pre-approval.