Good evening, @sgcanella.
It's great to see a new face here in the Community. I'd be more than happy to answer your question.
Please know that QuickBooks does not currently offer after-tax Roth catch-up limits. To set up the said contribution correctly, you need to follow the details and the annual limit displayed in QuickBooks. I also recommend seeking an accountant's advice to address this situation. They can provide guidance on how to manage this catch-up contribution, particularly on how to set it up in the program.
Furthermore, keep in mind that QuickBooks is continuously improving the product's features to meet our customer's needs. With this, I suggest going to the Gear icon in QuickBooks Online and you're able to choose Feedback from there. This way, you can submit a request about having the option to manage After-tax Roth catch-up limits for the retirement plan in QBO. Any recommendations are sent to our engineers for consideration in future updates.
Reach back out if you have any further questions. I'm only a post away. Take care!