You can't get the trial if you already have an active membership. Note that each account may only claim one free trial. The Free Trial offer applies to the entire Nintendo Switch Family, including the Switch Lite and Switch OLED models.
After that point, auto-renewal will automatically activate for an individual 30-day membership.
Sign up for a seven-day free trial of Nintendo Switch Online either by using your Switch or on the website. See the Warranty link in the site footer for other limitations or exclusions. Note that the warranty does not apply if damaged by non-Nintendo products, unauthorized modifications, or for products used for commercial purposes. If you need to make a claim, contact Nintendo’s Customer Assistance Hotline at 1-80. Hardware is covered for 12 months and games and accessories for three months, with both eligible for repair or replacement if an issue occurs within those periods after purchase. Aside from the NES and Super NES Classic Editions, all Nintendo products are protected by a limited warranty that ensures a product will be free of material and workmanship defects for a specific period.