@yuhui
Please clarify what are trying to achieve?
API COMMUNITY SERVER
POST api/2.0/authentication
https://api.onlyoffice.com/portals/method/authentication/post/api/2.0/authentication
[1] 
[2]
This link may be useful to you
https://forum.onlyoffice.com/t/get-file-with-api-or-wopi/3500/5