Follow these steps to set your custom Frontpage (or any other URL) as the default "New-Tab"-Page in your Safari Browser:
1. Open the Safari Settings
Open Safari on MacOS and click on "Safari" in your Menubar. This will open a drop-down menu. Select "Preferences" to open up the Settings-window.
2. Change the New-Tab page
In the Safari settings, click the "Generals" tab and click on the drop-down menu next to the label "New tabs open with". After selecting "Homepage" from the drop-down menu, enter the URL of your Frontpage, e.g. "https://myfront.page/example" in the "Homepage" input-field and you are ready to go!
You can now close the Settings-window. If you open up a new tab in Safari from now on, you personal Frontpage will appear! Awesome!
If you wish to reverse those changes and want to see your default Safari "New-Tab"-Page again, repeat the above steps but instead of selecting "Homepage", select "Favorites", "Top Sites" or "Empty Page" - whatever your previous configuration was.