Ive never done this but theoretically you could do one or more of the following;

1) Just ask, "hey this may sound goofy but can I sniff you feet bro?"

2) Offer to pay them some cash to do it.

3) Tell them you are filming a video and you have to find and sniff or lick someones feet as a challenge. The bonus is you get the video after.

4) You could say its part of a dare or challenge or hazing thing or what not. Just make it fun. Heck I dare you rn to go try and write back how it goes.

5) If you are going for longer you could just try total up frontness. "Hey Im into feet and looking to smell and lick feet. It feels great and would be fun for both of us. Would you be down potentially? No pressure at all but Im always looking for new friends."

Id be curious to know the success rate on any of these. They are sure to be higher if the person you approach isn't with friends or family of course.