Typically there is a menu to the left of Machine with the label File, and under the File menu there should be an item with the label Preferences. That should open a dialog box with a eight or so groupings on the left: General, Input, Update, Language, Display, Network, Extensions and Proxy. You will want to choose the Input grouping and there you can assign an alternate host key.
Alternatively, you can also use the VBoxManage command line tool with the commands getextrametadata / setextrametadata to retrieve and change the global host key configuration:
VBoxManage getextradata global GUI/Input/HostKeyCombination
VBoxManage setextradata global GUI/Input/HostKeyCombination <key_constant>