Enable grub boot menu from window 10

Execute the following command from Windows 10 in command prompt (run as administrator)

bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

Execution Logs for reference:

Microsoft Windows [Version 10.0.14393]
(c) 2016 Microsoft Corporation. All rights reserved.

C:\WINDOWS\system32>bcdedit

Windows Boot Manager
--------------------
identifier              {bootmgr}
device                  partition=\Device\HarddiskVolume1
path                    \EFI\Microsoft\Boot\bootmgfw.efi
description             Windows Boot Manager
locale                  en-US
inherit                 {globalsettings}
default                 {current}
resumeobject            {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
displayorder            {current}
toolsdisplayorder       {memdiag}
timeout                 30

Windows Boot Loader
-------------------
identifier              {current}
device                  partition=C:
path                    \WINDOWS\system32\winload.efi
description             Windows 10
locale                  en-US
inherit                 {bootloadersettings}
recoverysequence        {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
recoveryenabled         Yes
isolatedcontext         Yes
allowedinmemorysettings 0x15000075
osdevice                partition=C:
systemroot              \WINDOWS
resumeobject            {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
nx                      OptIn
bootmenupolicy          Standard
C:\WINDOWS\system32>bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi
The operation completed successfully.
C:\WINDOWS\system32>bcdedit

Windows Boot Manager
--------------------
identifier              {bootmgr}
device                  partition=\Device\HarddiskVolume1
path                    \EFI\ubuntu\grubx64.efi
description             Windows Boot Manager
locale                  en-US
inherit                 {globalsettings}
default                 {current}
resumeobject            {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
displayorder            {current}
toolsdisplayorder       {memdiag}
timeout                 30

Windows Boot Loader
-------------------
identifier              {current}
device                  partition=C:
path                    \WINDOWS\system32\winload.efi
description             Windows 10
locale                  en-US
inherit                 {bootloadersettings}
recoverysequence        {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
recoveryenabled         Yes
isolatedcontext         Yes
allowedinmemorysettings 0x15000075
osdevice                partition=C:
systemroot              \WINDOWS
resumeobject            {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
nx                      OptIn
bootmenupolicy          Standard

C:\WINDOWS\system32>