Is there any way I can apply the 25% TurboTax discount that I forgot to use when I filed my taxes and purchased my TurboTax product?

No one in the user forum can resolve a billing issue.  If you have a question about your TurboTax fees or billing, make sure you use the word “billing” in your request for help.  Do not use the word “refund.”

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

There is no way to apply a discount once you have paid for TurboTax.  It may be possible to pass the link to a friend if it is something that the friend would also qualify for.