A few lessons learned from going out with MF couples.
Write an ad geared for the woman. If a guy gets permission for a MMF or MFM then he's probably going to have to hunt for a prospective third and show his wife the ad that he's responding to. She always has veto power. So write something that he won't be embarrassed to show her. Write something that will turn her on.
Start with a photo and build a story around it. I'd suggest something titillating or flirtatious but not pornographic. Maybe a nude while sleeping on your stomach. Or sitting on the patio drinking coffee with a cup covering your privates. Or at a nude beach with a beach-ball. Leave your privates to the imagination. Enlist the help of a friend to take a few shots.
Take time writing and rewriting your ad. Even a good one might not get any responses at first. Again, remember titillating and flirtatious not pornographic or crude. Is there an animal you identify with? Maybe write something about how you are a pet seeking a loving couple.
There are a lot of guys who post ads to try a three-way that couples can respond to so be patient and re-post it a few times. Eventually you will get a few quality bites.
Arrange dinner in a public place where you can talk and get to know each other and plan to hit a dessert shop after. The restaurant should be some place that she will like the most.
Let the husband decide who sits where. For example if it's a booth he may want to sit on the same side as his wife, or he might get turned on by your sitting next to her. My vote is to sit across from her so that you can look into her eyes while you talk.
Make an effort to pay attention to her. Ask her lots of questions, get to know her and most importantly flirt with her. Remember, the woman almost always has veto power and generally women want to connect with someone they are going to sleep with. This is especially true if you want something ongoing.
Leave the couple alone at least twice during dinner. They will need to check in with each other and your absence will let them do that. Remember that protecting their relationship ought to be paramount to them and give them some space to do that.
When the check comes be prepared to pay the whole bill, this is a date after all. The husband may insist, remember it is his duty to be the provider. Politely, offer to pick up the bill three times. If he still insists then you say OK, but let me treat the two of you to dessert. You ought to have two or three different kinds of places in mind. This gives you the opportunity to contribute to the evening, to spend more time getting to know them, and to win his heart (because the way to a man's pants is through is stomach). But, vote with whatever place the woman wants to choose.
After the dessert you can walk them back to their place or their car. If you really want to see them repeatedly then it's probably a good idea not to have sex on the first date.
The husband is probably the one you've been chatting with online, so after the date write him about what a great time you had, how hot you found the two of them and how you hope to take it a step further. Let him know that you wanted to make his wife feel as comfortable as possible and that you thought he was hot too.
Repeat for the second and third date and throw in a movie, show or stand up comedy. Find excuses to hold her hand or touch her arm and to put your arm around his neck in good old buddy fashion or even give him a little shoulder squeeze/rub. Eventually the idea of a date night at home will come up.