I don't know of a way to do this, but would love to be able to add a popup message when a map is opened in Collector. Often times there are certain prerequisites that need to be completed before data collection can start. For instance, we've deployed high-accuarcy GPS data collection through Collector and I need to make my users aware that they have to set the Provider and change their Profile before they can start collecting data. Other times they need to go out and use a map they haven't used in a year (certain inspections we only do once a year) and it would be great if I could provide a link to a how-to document for that specific task when the map is opened. I know I can add information in the Description section when setting up the Web Map in Online which can then be viewed in Collector under Map Details but a popup when the map is opened would be a lot easier for my field users.