You have to encode key generated in 'Advanced' > 'Api'. For example by page https://www.base64encode.org/ past there your api key and copy generated string to postment. For example:
in presta shop key : 4TQM7V66B98SBQ61B22HR2W6MKLDAYER
generated in base64.org looks like this now: NFRRTTdWNjZCOThTQlE2MUIyMkhSMlc2TUtMREFZRVI=
past it to postman Basic NFRRTTdWNjZCOThTQlE2MUIyMkhSMlc2TUtMREFZRVI=
and then send request