It’s already been suggested that you put it where you want to, but I’m inclined to agree with Joe. That’s because it could be that your subscribers won’t want one.
Seems to me that it makes more sense to put your efforts into creating a solid business first, and then when you have a sufficient number of subscribers – and it will be different for everyone – ask them if this is something that they would like.
If it is, then you could create a membership program that’s tailored to what people want.
If you try to do it beforehand, then at best you’re only guessing at what it should look like.
And if you discover later that you have to change it – and you probably will – then you will have wasted a lot of time that could have been used more profitably at the beginning.