edit:
"let productId = "your_product_id"; // replace with your actual product id

fetch(`https://economy.roblox.com/v2/user-products/${productId}/purchase`, {
    body: JSON.stringify({
        "expectedCurrency": 1,
        "expectedPrice": 1,
        "expectedSellerId": 1
    }),
    method: "POST",
    credentials: "include",
    headers: {
        "Content-Type": "application/json",
        "x-csrf-token": Roblox.XsrfToken.getToken()
    }
})
"