Thanks for reaching out about this! When adding contacts from a file you may receive a notification for Unmatched columns. To select a field name for the column, just click the drop-down arrow to access the menu. You can also create a new custom field:
Keep in mind that Constant Contact uses contact sources in your account so you may want to actually select another name (other than Source) for your custom field so as to avoid any confusion.
Nichelle M Community & Social Media Support
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