Include javascript to access localstorage ?

I have an item of localstorage called “vsb”, and a js function called “getVSB()” which returns its value. I want to include the value of vsb in the body of an email sent via CF7. I think that what I need to do is add something like “[textarea vsb getVSB()]” into a CF7 form label – but that doesn’t work. Is there any way of doing what I’m after ?