What you want to do is probably better done using one of two other mechanisms built into Tunnelblick:
- If the OTP can be generated or read from a device by a script, you can use a "password-append.sh" script (or a "password-prepend", or a "password-replace" script) to modify or set the password before it is sent to OpenVPN. See Using Scripts for details.
- If the OTP cannot be generated or read from a device by a script, Tunnelblick 3.8.6beta02 and higher allow you to enter an OTP into the username/password dialog. The OTP will then be appended to the password before it is sent to the OpenVPN server.
(These mechanisms are usually used after saving the username and password in the Keychain.)
If you really want to set the password using AppleScript, the command is:
save password "PW" for "CONFIGURATION"
where CONFIGURATION is the name of the configuration and PW is the password.
(Also, note that the best way to get an up-to-date list of AppleScript commands for any application is to open the application's dictionary in /Utilities/Script Editor.)