Enable GRUB Menu, Disable direct start-up to windows

 

• Log into Windows and make sure fast startup is disabled by right clicking the start menu and select Command Prompt (Admin) option. Now type in powercfg -h offand press enter.
• Type the following into the command prompt and press enter:

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

• Restart your computer and you should now see the Grub screen!

Source: https://medium.com/@mugglestudies/common-problems-with-ubuntu-and-windows-dual-boot-installations-the-official-muggle-studies-guide-653fa37116b2