To obtain your User ID you'll have to navigate to your Shopify Settings and click on Users and permissions.

Once you're in the Users and permissions settings you'll see the Store owner and a list of any Staff. For this example we've simply changed all of the names to John Doe for privacy.

Click on any staff whose User ID you'd like to obtain. You can also click on the Store owner's name to find their User ID the same way.

After you've clicked on either a staff member's name or the store owner's name, you'll be brought to the following screen below.

The number you're looking for is at the end of the URL and will be directly after account/. Make sure not to include the slash when copying your User ID.