Postman generates some Headers automatically. In UiPath you need to specify what needs to be sent.
In addition to checking your headers, see how your JSON standard is doing, in Postman you need to use the quotation marks (“) and in UiPath you need to send according to what your destination needs. (Some destinations accept that the quotation marks (”) are replaced by single quotes (').
Please refer below thread which is having more details
fvalencia
hentsou_mamy