Hello Will,
This can be completed with the following workflow:
1) Open the Collector web map
2) Click the magnifying glass for the Search tool
3) Enter the long/lat in the search in that order (not lat / long)
4) This will find the location and show a nice little pin with a popup box. If it does not find it, you might not be using the World Geocode Service as your organization's default utility geocoder for the ArcGIS Online/Portal for ArcGIS organization.
5) You then click the three dots or action button in the top right corner of the popup > a menu appears > Collect here
6) This brings you to the menu to pick which layer or feature type you are trying to collect
7) Enter your attribute values and submit - all done!
Let me know if there are any questions! This was tested with Collector for Android 10.3.7 but same workflow for iOS except the ellipsis is replaced with the iOS action button.