Link to the Qubes Docs: https://www.qubes-os.org/doc/dispvm-customization

This describes how to create an own disposable AppVM which will be based on an existing template

# Create a new Disposable App-VM which is based on a custom template (here named: t-fedora-26) 
qvm-create --template t-fedora-26 --label red --property template_for_dispvms=True --class=AppVM my-dvm
 
# TEST: Start an application in this dvm
qvm-run --dispvm=my-dvm xterm
 
# Fix menu entry from Domain: my-dvm to Disposable: my-dvm
# https://groups.google.com/forum/#!msg/qubes-users/gfBfqTNzUIg/sbPp-pyiCAAJ
# https://github.com/QubesOS/qubes-issues/issues/1339#issuecomment-338813581
qvm-features my-dvm appmenus-dispvm 1
qvm-sync-appmenus --regenerate-only my-dvm
 
# Change the Disp-VM from an AppVM (here for an AppVM which is named my-untrusted)
qvm-prefs --set my-untrusted default_dispvm my-dvm
 
# Try to start something from this AppVM in a disposable VM
qvm-run --auto my-untrusted 'qvm-open-in-dvm https:/google.de'
# This should start a new dispvm which is based on your dvm-App
# Check the template on which the dispvm is based on in dom0
qvm-ls | grep disp
 
# Set default DispVM
qubes-prefs --set default_dispvm my-dvm