When looking for an API call that deals with the Google Drive folder permissions, the API call files/{id}/permissions can be used also for folders as follows:
1. Retrieve the id of the folder through the call GET/folders/content.
2. Copy/paste the id in the POST/files/{id}/permissions API call and use the below payload:
{
"role": "reader",
"type": "anyone"
}
The above payload adds the reader permission with anyone with the link.