How Do I Find My Merchant Center Id?

Google Merchant Center is the central location for all of the product data used to drive your Google Shopping campaigns. Each Merchant Center account has a unique identifier, which is used when linking it to your AdWords account.

You can find your Merchant Center ID by:

Logging in to Merchant Center
Your account ID will be shown in the upper left corner

Next Steps
Input your Customer ID into  Step 4. Build Settings.