Nowadays, it is becoming common among the couples to try something different to make their sexual life thrilling. Cuckolding is now gaining more popularity. Many conventional girls, who are utterly against this, is slowly turning out to this fantasy and later on to reality with their husbands influence.
I can suggest some methods how to achieve this.
First and foremost, please do not forcefully inject this idea. Slowly and in a planned way, try to push this idea of a third person or another couple. If you do it during your love making session and prove that, you get much excited with that, your wife may get a point that, it will make you much happier. So, she might understand that it is your weakness.
Similarly, you can understand her stand or position on this. At first she may refuse, but, slowly she may accept this in fantasy stage. Later on, increase the adult jokes with her comparing yourself with other guys, their tool etc.
She might have some weakness of some film stars, sports stars etc. You can use advantage of that if you find someone like that. While love making, bring him in as a fantasized personality. Look to your wife's reaction.
This are some tips you can try. Wishing you all the best.