I'm posting the answer from the support ticket here as well:
Here's how you should add the field to the template, so it can be bound properly:
The name of my field here is "testCustom" and it is added in the basic profile. If you add the field it to another profile type, you will have to change the DataItemType to the relevant Profile Type. The DataFieldName property indicates to which field the value should be bound. Also, an ID is necessary to be added.
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