Accessing the plus API pool
get_cr_token(service = "https://api.crossref.org", ...)
| service | Service name, a character scalar.  | 
    
|---|---|
| ... | Arguments passed on to  
  | 
    
get_cr_token: Get the token to authenticate into the plus tool.
In this order, returns the first hit of:
CR_MD_PLUS_TOKEN environment variable
(recommended only for secure environment variables in the cloud),
an entry in the OS keychain manager for service and username,
NULL with a warning.