Our Merchant Promotions Add-on for Google Shopping allows you to set promo IDs on products using Modify Products.
After purchasing the add-on, use Modify Products to build your rules:
Start by selecting Promo IDs as your attribute and then click Add Rule:
Next, your rule type should be set to Text Attribute Builder where you will enter the promo ID set up in Merchant Center:
If you are only adding this ID to specific products, you can use a filter to determine which one's (If applying to ALL PRODUCTS, do not use a Filter at all):
For Product Channels, make sure to only have Google selected and then click Save:
You can use the preview table at the bottom to ensure your IDs are implemented correctly, however to finalize any changes you must send your feed to Google. It may take up to 2hrs for Google to process your modifications.