You can deploy FortiGate using custom templates or PowerShell from VHD image files.
VHD image files are available from Fortinet Customer Service & Support. Go to Download > VM Image, then select FortiGate as the Product and Azure for the Platform. The file name is FGT_VM64_AZURE-v6-buildXXXX-FORTINET.out.hyperv.zip, where XXXX is the build number.
Once the download is complete, unzip the file and locate the fortios.vhd file. Upload the fortios.vhd file to your blob/storage location as required by your deployment templates.
At a given time, Fortinet Customer Service & Support hosts only the two latest major versions with two minor versions each. To obtain older files, navigate to Download > Firmware Images, select FortiGate as the Product, then go to the Download tab. Go to the desired version and download the FGT_VM64_AZURE-v6-buildXXXX-FORTINET.out.hyperv.zip file.