In this sample script, all pages in the Figma file are retrieved and the retrieved pages are put to new Google Slides as the image.
You can see the method for retrieving the access token at here. Although there is also OAuth2 for retrieving the access token, in your situation, I thought that the method for directly generating the access token on the site might be suitable. So in this answer, the generated access token on the site is used. Please retrieve the access token as follows.