I believe you’re not constructing your POST request correctly, try smth like:
URL: https://api.pushover.net/1/messages.json
TYPE: plain/text
BODY: token=/pin[0]/&user=/pin[1]/&message=/pin[2]/
Not sure whether this pushover server will handle plain/text
type (most servers do), anyway I submitted a ticket to explicitly add application/x-www-form-urlencoded
type: https://github.com/blynkkk/blynk-server/issues/701