(Please also see the intro video "Adding a Linked Album to a Collection" in the Help category: Introductory Videos. This video uses Google Photos but the general concept is similar. Specific steps for Apple are found below.)
To Create a Shared Album
First, here is Apple's Instruction page for iCloud Shared Photos.
Our Direct-to-the-Point Instructions
Apple iOS 12 "Photos" allows us to create two types of presentations directly from our phone or computer. One is the Shared Album. The other is the Apple Slideshow/Movie, discussed in the next Help topic.
Be aware that iOS 12 Photos has two types of "Shared Albums". One is for the temporary sharing of photos amongst friends, and links to these Albums last only one month then disappear. The second type is a Shared Album that we believe will last as long as you have iCloud (and as long as Apple doesn’t remove it, which we see no reason for happening.) We, of course, want the second one.
Important: To create Shared Albums you must:
Be signed in to iCloud
Have enabled both iCloud Photos and Shared Albums on your device. Go to: Settings > Photos > a) iCloud Photos to ON; b) Shared Albums to ON
Creating a Shared Albums
An iCloud Shared Album is the combination of individual photos and videos into an Album that can then be shared and viewed item by item. iCloud Shared Albums remain stored in iCloud.
On an iMac or Macbook
Click on Shared Albums (left column) and tap the "+" sign to the right
Enter a Shared Album title; (no other info needed); select Create
Now double-click on your new (but empty) Shared Album in the list of Shared Albums.
Select Add photos and videos and select the photos and videos to add to this Album; select Add. (It may take a few moments for photos in your new album to appear.)
KEY STEP: At the top of the screen, click on Circled Person icon and turn on Public Website
Now drag your cursor to copy ([command][C])the Share Link. You now have the link to this new Shared Album that is stored in iCloud that you can insert into a Collectionaire collection.
On an iPhone
Tap the Albums tab at the bottom of the screen
Tap the “+” sign in the top left of screen & select New Shared Album
Enter a Shared Album title; select Next
Select Create (no need to enter a person/address for "To:")
Tap on this new but empty album space, then the “+” sign to add photos or videos to the Album;
Tap photos to get a check mark for those selected, and click Done
The cursor is giving you the option of adding a message. No need to add a message and just select Post. (It may take a few moments for photos in your new album to appear.)
KEY STEP: At the bottom of the screen, click on People tab and turn on Public Website. A share link will soon appear after the app finishes "Publishing...". (This sometimes takes a while and you can come back to the shared album and get the link later).
On this screen you can also invite others to contribute to the Shared Album.
Select the Share Link and the Copy option. You now have the link to this new Shared Album that is stored in iCloud.
To copy the link, you can just click copy and you’ll have the link, or you can email it to yourself, which works well if you are creating the album on your phone but uploading links on your desktop to enter into Collectionaire.
Note About Preview Image
Unfortunately, Apple Photos does not provide a cover photo from which Collectionaire can generate a preview image. But here's how to get one quickly (For other ways, please see Help title: How do I add a Custom Preview photo?
On a Mac
In the New Linked Album page, click REPLACE ALBUM PHOTO under the Album Link
Select Photo Library, your Shared Album, the preferred photo, and Save
On a Windows PC
Open your iCloud Shared Album
Select the photo you wish as your preview image; click Download. The image is now in your computer's Download folder
Click Replace Album Photo under the Album Link.
Select the photo from the Download folder.
Also helpful from the website imore: