Per this post, add reimbursable expenses to the web version of Business Pro. Also update this help article to make it clear that it isn't currently available.
Please, please, please don't put this on hold. This feature is a game-changer when it comes to billing, it saves so much time. I mean I can still continue using it in the desktop version instead, but I'm finding it hard to train new staff to use certain great features in the web browser (MYOB Business) version and then have to switch to Desktop to do other functions. It would be so cool if the Dev Team could succeed at being able to link Job-related Expenses to a Customer.