If you use the interface it lists the volume that will copy:

If you want to grab the corresponding Volume name you can use vssadmin:
vssadmin list volumes
vssadmin 1.1 - Strumento da riga di comando di amministrazione Servizio copia shadow del volume
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Percorso volume: \\?\Volume{16aa3286-4a3d-11e3-824b-806e6f6e6963}\
Nome volume: \\?\Volume{16aa3286-4a3d-11e3-824b-806e6f6e6963}\
Percorso volume: C:\
Nome volume: \\?\Volume{16aa3287-4a3d-11e3-824b-806e6f6e6963}\
Percorso volume: \\?\Volume{b80c588e-0000-0000-0000-a0694a000000}\
Nome volume: \\?\Volume{b80c588e-0000-0000-0000-a0694a000000}\
Then you can try composing the command line:
disk2vhd \\?\Volume{16aa3286-4a3d-11e3-824b-806e6f6e6963}\ C:\ c:\temp\Win10.vhdx
Never done myself, just a try..
HTH
-mario