I purchased the apple tv 4k from costco on 5/29/23, but was unable to get the 3 months free. How do i contact someone to get it?

Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support

Note that you can normally only take advantage of a trial that you have never had before.