I regret closing the window before starting this reply because I remember the basics from the question but not all of the details. If you were the one to leave the message, please leave a comment here and I can try to address the specifics more directly.
Basically, if memory serves me, the writer was a sub in a long-standing marriage with some Femdom tones that was wanting his wife to have him wear a bra and panties and his request was denied and he was looking for advice on how to convince her of the idea. (also I think there was a mention of chastity but I don't remember the specifics.)
I have written about this topic several times in the past but I will try to reiterate some of the more important points. What you need to "sell" her on is how will it benefit her. Basically, by forcing you to dress in a bra and panties, what does that add to her life, your relationship, and so on? Will her life be better because of it or is it just something you want to do? Will it justify its cost?
You need to be able to answer these questions and be willing to make extra sacrifices to make it happen (assuming it is truly important to you). You can try tackling them one by one.
"When you have me wear a bra and panties, it makes me feel more submissive and more loving. I also feel more obedient and would be willing to do
"I would be willing to give up
"It is something that touches my submissive soul deeply and I think I would be able to make you happier because
If you can't answer these questions you might have to just accept that it probably won't happen, beg and plead and hope it does something, or hopefully win a bet of some sort where that can be the reward.
The other compromise (after finding ways it will improve her life) is to seek a trial basis of it. See if she notices a difference over two weeks and let her decide after that. Unfortunately, if she's adamant about it you might be out of luck.
Here are some links to posts that I have made that have dealt with similar subjects:
Unfortunately there's no sure-fire way to sell it to her. The best you can do is to come up with a compelling argument and hope that she will be willing to either buy into it or at least entertain it on a trial basis.