Thanks for joining the thread, Adam-Ads.

Adding a purchase on the QuickBooks Online mobile app is beneficial to you and any other business however this option can only be access on a web browser either on a computer or mobile device. 

Feel free to send your valuable feedback by going to Gear icon and select Feedback. 

Your valuable feedback goes to our Product Development team to help improve your experience in QuickBooks. 

Also, you may check on this article on how to create and send purchase orders in QuickBooks Online. 

You can get back to me by commenting below if you have any other questions about purchase order. I'm always right here to help you.