In order to set up the YouTube Complication, you need an API key for YouTube, and the channel you want to show the subscriber count for's Channel ID. Both of these are free and easy to get.
Enter a Project Name of your choice, this guide will be using "YouTube for Smartspacer", and click Create.
Open the hamburger menu if it is not already open and select APIs and services > Library
Search for "YouTube Data API v3" and click the result
Click "Enable", you will be redirected to a page with a bunch of details on it
Click "Credentials" on the left hand side
Click "Create Credentials" at the top and select "API key"
A dialog will appear containing a long string starting with "AIza". This is your API key. Copy it.
Paste this API key into the settings for the YouTube Complication for Smartspacer as required, and save. You can now close the Google Developer Console tab.
Channel ID
Desktop
Navigate to the channel you want to get the ID for
Select the About tab
Click the share icon, and then "Copy channel ID"
Paste the channel ID into the YouTube Complication for Smartspacer as required and save. The UI should update to show the information for the channel you have selected
App
Navigate to the channel you want to get the ID for
Open the options menu and select Share
From the share sheet, select Copy Link
Open a browser of your choice, paste the link and go to it
Follow the Desktop steps above, from step 2. You may need to enable desktop mode in your browser to get the desktop UI.
Delete page
Deleting the wiki page "2: YouTube Complication Setup" cannot be undone. Continue?