Use a .htaccess or any type of redirect. You may upload these files using FTP client like Filezilla.
You may add [php][/php] to the head section of the “Comming Soon”. It will redirest users to http://wp.domain.com after 5 seconds.
You may also customise this time by changing the 5 to 0 (for instant redirect) or anything.
To do this you should download the file and using text editor add the code in before