No. Your full contribution is not available for use at the beginning of the year. You may only spend the funds that are available in your HSA at any point in time. For example, after each payday when your employer sends your pre-tax contribution to the HSA administrator, then those funds are available. If your employer does not offer payroll deduction, you may send an after-tax contribution to the HSA administrator, and then claim those amounts later when you file your annual tax return as an above the line tax credit, which reduces your taxes.
Find more HSA-related FAQs here: https://www.optumbank.com/tennessee/customer-support/faqs/hsa-faqs.html.