How do I add a custom filter to the collection page
In your shopify Admin panel, click on the "Filters" in the left sidebar menu
In order to add custom filter in your store.
Select "Selected collections".
After checked mark this, You can see the Selection box right below the selected collections section.
In this section, You can select custom collection.
You have the options to select collections as many you want
After done after setting, Click on the "Save Filter"
All done, Your filters will be save successfully.