Blurting out "I want you to be my Daddy," to your partner might not go over too well, especially if they are vanilla. The taboo around Daddy Doms comes from a lack of understanding what this dynamic is all about. Here is a link to an article from this site explaining more about Daddy Doms: Daddy Doms: They're Not What You Think.
If your partner is already friendly with this idea, you'll likely have an easy time getting such a dynamic established. Otherwise, it can take some warming up to the idea.
Regardless, you know your partner best, and can make the best call on how to bring this up. Here are some subtle ways you can go about introducing the concept:
1. Call him Daddy. Doing this during sex is probably one of the easier and safer ways to bring something up at first. It's hard to not like something when you're having an orgasm. You can judge your partner's reaction to help you figure out if this idea will be a hit or a miss with him.
2. Act a little. Sometimes playing the part you want will help bring out the Daddy Dom in your partner. If you take a subtle approach, it can help ease you into things, so it doesn't sounds like wanting your partner to be your Daddy is out of the blue, and maybe even take some of the taboo out of it.
3. Talk about your "fantasies." This is a way to express that this dynamic, having a Daddy, is something that you are interested in, in a more relaxed way.
If your desire is to make this into a lifestyle for the two of you, you will more likely than not have to have a direct conversation at some point. You'll want to figure out what this means to the both of you, and address the shift to this dynamic vs. it just being some bedroom fun. This could involve laying out rules or just a simple confirmation that you are both on the same page and going to let the dynamic evolve with time.
If your partner is initially a bit "grossed out," thinking this is perverted, incestuous, or that you have "daddy-issues", be ready to explain what the Daddy Dom and little dynamic is about and what it means to you.
In my relationship, we started falling into this dynamic without specific intention to do so. When I realized we were headed in that direction, I did my research on Daddy Doms and littles. I found that this was what I wanted. The closer we drifted towards it, the more right it felt. Our biggest hang up was on the term "Daddy." I knew my boyfriend was starting to fit that title more than anything else. But we both initially found it to seem a bit too incestuous, which is not appealing to us. What helped me get past that was thinking about other terms of endearments. Like at that point in time, my soon-to-be-Daddy was already calling me baby girl and baby and many more things. You those don't have to translate to imagine me as an actual baby. Consciously recognizing that terms of endearment don't exactly correspond to the word’s dominant meaning, but rather the kind of feelings they evoke, helped me get past the taboo of the term "Daddy."
With my boyfriend having already drifted into such a role before the application of the name "Daddy" and before we ever discussed the dynamic, it ended up being a smooth transition. I talked to him about it, and he seemed to go along with it at first just to make his baby girl happy. And now we live the lifestyle whenever we can, it seems like a perfect fit for the both of us. I've never been happier.
So my final word on the matter is: if you know what you want, go for it. Don't let the taboo of this hold you back.