Add variables to dropdown field through option expressions

Hi,
I am trying to show fields from a json in a dropdown field.
I am retrieving the json correctly through an Http task, setting Save response as JSON as true and storing it with the name “response”.
I get this information: [{“unitId”:1},{“unitId”:5},{“unitId”:6}] and I can show it in an expression as {response}, but I want to show each unidId value as an option of a dropdown field. I have enabled options expression in the dropdown field and have tried to retrieve these values with {response}, {response.unitId}, {{response.unitId}}.
Does anybody know how to do this? Thank you in advance!