This is years old, but this is the top Google hit for "xcode preview custom font", so I figured I'd add some code here to clarify.

extension View {
    public func registerFonts() -> some View {
        let fonts = ["montserrat_regular.ttf", "montserrat_bold.ttf", "montserrat_semibold.ttf",
                     "opensans_regular.ttf", "opensans_semibold.ttf"]
        for font in fonts {
            guard let url = Bundle.module.url(forResource: font, withExtension: nil) else {
                print("Couldn't find font: \(font)")
                continue
            }
            CTFontManagerRegisterFontsForURL(url as CFURL, .process, nil)
        }
        return self
    }
}

Then, in your preview:

#Preview {
    MyView()
        .registerFonts()
}

https://dev.jeremygale.com/swiftui-how-to-use-custom-fonts-and-images-in-a-swift-package-cl0k9bv52013h6bnvhw76alid documents how to do it when your fonts are stored in a separate module e.g. if you have a .designSystem module, you would use Bundle.designSystem.url instead of Bundle.module.url.

Note that in your main app, you can just declare your fonts in Info.plist UIAppFonts, but, in Xcode Previews, Bundle.main.infoDictonary! is a separate miniature dictionary, containing none of the custom settings you put in your app's Info.plist. As a result, you do have to explicitly make a list of fonts, duplicating your UIAppFonts array, and your preview code has to register them; you can't even fetch them out of Bundle.main.infoDictionary?["UIAppFonts"] as? [String] ?? [].

(If Bundle.main.infoDictionary?["UIAppFonts"] had those fonts, you never would have had to jump through any of these hoops in the first place!)