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I would like to know if it is possible to set the fallback of a custom field:
What I mean is, I think that it might be a good option to be able to set the fallback= at the time that the custom field is created so that from that time onwards the fallback will be used if none is supplied in the HTML when email are written.
It's kind of like setting a default at the time you create the custom field.
This is already supported. All custom fields need to be in this format (exactly what you presented as an example above), eg: [country,fallback=UK].
How to enter this 'default' at the time that you create the field?
You have to type in the fallback value when creating a campaign.
Another thing worth noting - the fallback value is optional. You can use a custom field tag in your newsletter code like this as well: [country,fallback=]. But you cannot use a custom field tag like this: [country].