Hello @Lydia-Lau and welcome to the Sound Forge group.
I am not familiar with Audio box but... - Go to "Options> Preferences> Audio" and select the 'Record' tab. - Confirm or select Audio box as the record device type , - Go to "View> Record options" and make any parameter changes if so desired. - Leave the "Record options" window open and select "Arm" (shortcut: Ctrl+ Shift+ A) - Viewing the record level meters in the Record options window, make any necessary record level adjustments in Audio box. - Commence recording by selecting "Record" in the transport (shortcut: Ctrl+ R) .
btw, Prior to recording the sample rate and bit depth can be changed by selecting "File> New". Typically, 44.1kHz for music and other audio only material, 48kHz is the standard audio sample rate for video. I usually recommend saving and editing in a PCM <.wav> file format. An MP3, AAC or other lossy format can be encoded from the PCM master for end users.
Besides the max 30 day activation, there may be other 'demo' limitations with Sound Forge.. For instance, the Vegas demo only allows rendering files up to two minutes. Not sure about SF. I don't think there would be any limitation with the device type settings.