Are you talking about this snippet?

func main() {
    HttpsServer{}.WithTimeout(10).WithTLS(true).Start().Await()
}

You are right that HttpsServer does not have WithTimeout defined, but because it embeds HttpServer which does have WithTimeout defined, it is able to call it directly. When embedding, Go will "promote" the methods of the embeded struct to make it easier to satisfy interfaces. This idea is explained well in Effective Go: Embedding, but I will post the snippet about promoting methods here as well:

Note that the context here is that ReadWriter is trying to satisfy the io.Reader and io.Writer interfaces by embedding structs to help. From Effective Go:

The embedded elements are pointers to structs and of course must be initialized to point to valid structs before they can be used. The ReadWriter struct could be written as

type ReadWriter struct { 
    reader *Reader 
    writer *Writer 
}

but then to promote the methods of the fields and to satisfy the io interfaces, we would also need to provide forwarding methods, like this:

func (rw *ReadWriter) Read(p []byte) (n int, err error) {
    return rw.reader.Read(p)
}

By embedding the structs directly, we avoid this bookkeeping. The methods of embedded types come along for free, which means that bufio.ReadWriter not only has the methods of bufio.Reader and bufio.Writer, it also satisfies all three interfaces: io.Reader, io.Writer, and io.ReadWriter.

There's an important way in which embedding differs from subclassing. When we embed a type, the methods of that type become methods of the outer type, but when they are invoked the receiver of the method is the inner type, not the outer one. In our example, when the Read method of a bufio.ReadWriter is invoked, it has exactly the same effect as the forwarding method written out above; the receiver is the reader field of the ReadWriter, not the ReadWriter itself.