VariableSource
contact (a contact detail), attribute (a custom field) or static (a fixed value).
sourcestringrequired
Possible values: [contact, attribute, static]
keystring
Example:
first_namefallbackstring
Possible values: non-empty and <= 200 characters
valuestring
Possible values: non-empty and <= 1000 characters
VariableSource
{
"source": "contact",
"key": "first_name",
"fallback": "string",
"value": "string"
}