Hi Sam,
When you include {keyword:default text} in the TextAd, the default text will be used when the text can't be replaced by a keyword. If you want to use default text in Ad Customizers in a similar manner, the syntax will be {=FeedName.AttributeName:default value}. Please see the
Customizing Text Ads guide for more details.
Regards,
Shwetha, AdWords API Team.