VBoxManage
We can issue any application on the guest from the host with VBoxManage guestcontrol. There are several options to this command nicely documenetd in the above linked Virtual Box manual.
As an example we may issue:
VBoxManage -q guestcontrol "Name_of_VM" execute --image "full/path/to/application" --username <me> --passwordfile mypassword.txt --wait-exit --wait-stdout [<-- options>]
This may be a security issue as your guest user password needs to be given in an insecure password file on the host.
SSH
Somewhat easier than this, and in case we had set up SSH to our VM we can of course access the guest application through an SSH session. This is how I usually access guest applications.
ssh user@virualmachine application