Access token for calling SKY APIs help

Hi,

Trying to get the access token so I can call SKY APIs.

Tried using the guide on the blackbaud website but had a lot of difficulties with that. I tried using the POSTMAN website recommended by Blackbaud's youtube channel as an alternative method. I'm getting this error.

Error: CORS request rejected: https://oauth2.sky.blackbaud.com/token

In the postman page (screenshot) I filled in the client id, client secret -as it was in my application in Blackbaud/RE - and the authorization and access token URLs. In my application in Blakcbaud/RE- I used the "Callback URL" in POSTMAN (in the screenhsot)

Please could I have some help.

Thanks