Enter your custom font name in "Your Font Here".

/// A modifier to change `Font`
/// - Parameters:
///   - size: Size of `Font`
///   - fixedSize: If set to `true` then the font size will not change dynamically. Default is `false`
/// - Returns: A view of size and font
struct SiennaFont: ViewModifier {
    let size: CGFloat
    let fixedSize: Bool
    let font = "Your Font Here"

    func body(content: Content) -> some View {
        content
            .font(fixedSize ? .custom(font, fixedSize: size): .custom(font, size: size))
    }
}

extension View {
    /// A modifier to change `Font`
    /// - Parameters:
    ///   - size: Size of `Font`
    ///   - fixedSize: If set to `true` then the font size will not change dynamically. Default is `false`
    /// - Returns: A view of size and font
    func myCustomFont(size: CGFloat, fixedSize: Bool = false) -> some View {
        modifier(SiennaFont(size: size, fixedSize: fixedSize))
    }
}

Then use it like if you want the text to change dynamically.

Text("Hello World!")
     .myCustomFont(size: 40)

or if you want it fixed size then

Text("Hello World!")
     .myCustomFont(size: 40, fixedSize: true)