There is a way to avoid the secured services sign in. Create an item in ArcGIS Online (or Portal) that points to your secured service. Choose "Add Item from web" and then enter the URL of your secured service. It will detect that it is a secured service and ask you for credentials and then if you want it to save those credentials.
Tell it to save the credentials, then save the item (it will be a "Map Image Layer"), then share it with the appropriate group.
Then go your your web map, choose "add" then "search for layers" (not 'add layer from web') and add the item you just created.
Now when you access the web map via Collector, it will log into the secured service for you.
GISP