Connects to the BMS or related server using a username and password. If these are not provided,
a pop-up window will prompt the user to enter their credentials. The function handles authentication
and stores the resulting access token internally for subsequent requests.
No return value. The access token is stored internally for future use.
Arguments
username
The username (optional, default is NULL). If not provided, the pop-up window is triggered.
password
The password (optional, default is NULL). If not provided, the pop-up window is triggered.
encoding
Specifies how the request body should be encoded: `form` (application/x-www-form-urlencoded),
`multipart` (multipart/form-data), or `json` (application/json). Default is "json".