How do I ask for refund for my iPhone 11?

Not likely Apple will issue a refund. 97% within 3 months is considered within expected operating parameters.

Also, this is a user to user forum. You are not addressing Apple, only other users like yourself. So we cannot grant you a refund.

Use the support link to contact Apple directly and see if they are willing to grant you a refund.

See here first: iPhone Battery and Performance - Apple Support

Maximum battery capacity measures the device battery capacity relative to when it was new. A battery will have lower capacity as the battery chemically ages which may result in fewer hours of usage between charges. Depending upon the length of time between when the iPhone was made and when it is activated, your battery capacity may show as slightly less than 100%.

Apple has a standard one year warranty for parts and workmanship. A warranty does not include a refund. There is nothing wrong with your iPhone but check the battery health one month prior to the expiration of your warranty and if it is at 80% or lower have them replace your battery.

You don't. The refund period for Apple products is 14 days. The Battery Health at 97% is not a problem.

I seriously doubt that you will get a refund because there is nothing wrong with your phone. If you are using the phone, it stands to reason that the battery level will decrease over time. I'm not a heavy user and my iPhone 8 stayed at 97% for a couple of years before recently dropping to 96%.