I've created a map in ArcGIS Online and enabled the "Find Locations > By Layer" settings, as per this article:
Configure feature search—ArcGIS Online Help | Documentation
I've also ticked the "Use in Collector for ArcGIS" option.
Now, in ArcGIS Online, I can search by location (just giving it an address), or by various details such as site name, ID etc.
However, this has not carried over to Collector. Whenever I search it is only by location/address and not by any of my layers. This is a great shame as we cover a huge area geographically and it would take a lot of usability out of Collector if our guys on the ground had to navigate manually (there will also be a lot of times where GPS probably won't be usable).
I know Collector is taking notice of this - if I remove the "By Address" option then it isn't able to search through anything. I'm using regular Collector (not Classic) on an iPad.