If you need a quick solution and don’t have time to buy a racerback clip you can make one from a couple of paper clips. Racerback clips are pretty cheap and will sit flatter under your clothing, so if you are going to use it often I would suggest buying one. But if you need something in a hurry this is a good option.
Put them on like in the picture above, use at least 2 as one is likely to bend. Tug at it to make sure it won’t bend open, add another clip if you think it will. Use paperclips that have smooth ends to the wire, otherwise it could scratch you. Twist ties are also a good quick fix, or you can use string to tie the straps together.