I have seen a video that someone used a custom font for a custom backpack
Video: - YouTube
I don’t think there is a feature yet where you can add custom fonts to Roblox Studio.
if you play the game bedwars on roblox and join a game you will see there is a billboardgui text with a dif font
They used the LuckiestGuy font. That font is already in Roblox Studio and not custom.
And if you want to use a custom font, you have to create it with 3rd party software and upload the text as a decal, then use an image label, and change that image.
You can go to google drawings and create a new text. Then choose a font from there. And export it as a png, after that make it a decal.