I would like to launch a Collector web map from an external application. This particular webmap allows offline areas.
When I use the url
Sorry for the late reply. Have you tested this in Field Maps? The error message may be expected. When there are multiple areas, there is no way to identify which one to open, as the URL is only passing the parent web map ID (there is no way to specify a specific area ID). Try opening the area you want, then following the link again. It should take you to the area you want and proceed with any other parameters that were passed.
In short, the area needs to either be the only offline area for the given map or the "Current" map. Does this method work for you?