The dp.txt file needs to be present inside the guest OS.
Why don't you first copy the file over with a Copy-VMGuestFile cmdlet, and then use an absolute path to access it ?
For example something like this:
Copy-VMGuestFile -Source dp.txt -Destination c:\temp\ -VM $vm -LocalToGuest -guestuser administrator -guestpassword xxxxx
invoke-vmscript -vm vm -scripttext "diskpart /s C:\Temp\dp.txt" -guestuser administrator -guestpassword xxxxx