the following request will return the Contacts record with contentid 123456:
Is there a way to use a field instead of the contentid, e.g. username which is a textfield?
Not through the content API. What you are describing is an Advanced Search, which can currently be accomplished only through the SOAP API.
Please take a moment to upvote the idea on the exchange that calls for feature parity between the available APIs.
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Thank you Bodie for you quick response. I will certainly vote for this idea.