Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
The syntax for "cf:custom_field_name" requires that all letters be in lowercase and spaces be replaced by underscores for the name of the custom field.
If you modify your fields in the JSON request body like below, you should see them populate correctly in the account:
For additional reference: "Custom Field 1" would be "cf:custom_field_1".
Please have a look and let us know if you have any other questions!