I'm setting up a headless shopify store and trying to get back in stock notifications working, but I'm having some trouble

My fetch request looks like this

fetch(`https://a.klaviyo.com/api/v1/catalog/subscribe`, {
    method: "POST",
    headers: { "Content-Type": "application/x-www-form-urlencoded" },
    body: new URLSearchParams({
      a: process.env.NEXT_PUBLIC_KLAVIYO_KEY,
      email: email,
      variant: properties.variantId,
      platform: 'shopify'
    })
  })

but the issue is that my shopify variant Ids are alphanumeric and this endpoint is requiring the integer ID that comes from my klaviyo catalog, and I’m a bit stuck on how to get this working properly.

Any info would be great, thanks!