At this time, the option to allow clients to send service request through QuickBooks is unavailable. I can see how this option would be helpful for you and your business. In the meantime, QBO does offer the support of thousands of various third-party applications for any functionality you may need. To see if there's an app that can help you accomplish this, head to the Intuit App center and type a keyword in the search-bar.
Lastly, I'd like to let you know that our developers are constantly uncovering and considering new ways to improve the program. To be updated with our latest news and enhancements, you can visit our QuickBooks Online Blog.
Stay in touch if you have any other questions or concerns. I’ll be right here to answer them for you. Enjoy the rest of the day.