Hey @agirlnamedS, some excellent advice already.
I’d say just be really clear on your boundaries, and your partner should respect that. Pleasing other people can be a huge turn on in itself, but do make sure it’s something you want to do as well. I’d recommend experimenting with them by yourself during solo play initially so you know how it feels, what works for you, how much lube you need (always more than you think), and what actions make it pleasurable for you. You can then introduce it into partnered play. If you’re feeling nervous when it comes to partnered play then I’d put it in by yourself first before sex, so you can do it in your own time and feel more relaxed.
I love butt plugs and beads during sex, rimming and fingers, but also not a fan of larger toys/anal penetration - it’s just one of those things that isn’t for everyone so please don’t feel like you have to do it if it doesn’t give you pleasure. I enjoy butt plugs during sex as they narrow your vaginal wall, and so can help to direct the penis towards your G-spot, and just give a really nice full feeling.
I love the jewelled metal butt plugs, and have similar to the one you’ve been looking at. I would however be inclined to recommend a silicone one for your first - they give a bit more movement whereas the metal ones (and glass) are rigid. They do feel amazing but I’m glad I worked up to it as they feel bigger, and give more of a “full” feeling. There are some silicone jewelled ones - these come with two different sizes, so you could see which you prefer/build up if you wanted to, but you could also have them as his and hers.
For all anal toys it’s really important to have a flared base. I also find for use during partnered play the ones with smaller necks are more comfortable. Position wise, anything goes really. Face down doggy and reverse cowgirl where your hands are on his ankles/face down will give him a wonderful view. Laying on your side with him behind also works. As others have said, sometimes just knowing that it’s there is a huge part of it, and he will probably be able to feel it as well.
I mentioned earlier, but don’t forget lube. If you go for silicone ones you’ll need a water based one. When inserting apply to the toy and yourself, go in as far as feels comfortable, take back out and apply more lube, and keep repeating Take your time with it and don’t rush.
Most importantly, enjoy a new experience!