+page.server.js

export actions = {
 prepyment: async ({ request }) => {
    const formData = await request.formData()
    const price = formData.get('price')
    const email = formData.get('email')
    const url = await generateCheckout(price, email)
    return { success: true, url }
 }
}

async function generateCheckout(price: number, email: string) {

			const res = await fetch('https://api.lemonsqueezy.com/v1/checkouts', {
				method: 'POST',
				headers: {
					Accept: 'application/vnd.api+json',
					'Content-Type': 'application/vnd.api+json',
					Authorization: `Bearer ${env.LEMONSQUEEZY_KEY}`
				},
				body: JSON.stringify({
					data: {
						type: 'checkouts',
						attributes: {
							preview: false,
							custom_price: price,
							checkout_options: {
								embed: true,
								discount: false,
								subscription_preview: false,
								button_color: '#C148AC'
							},
						},
						relationships: {
							store: {
								data: {
									id: '30883',
									type: 'stores'
								}
							},
							variant: {
								data: {
									id: '85474',
									type: 'variants'
								}
							}
						}
					}
				})
			});
			const json = await res.json();
			const { data } = LemonSqueezyCheckout.parse(json);
			return data.attributes.url
}

+page.svelte

<script>
  let paidForm
  
  const onGetPaymentUrl = () => {
    return async ({ update, result }) => {
			if (result.type === 'success') {
				console.log(result);
				const url = result.data.url;
				LemonSqueezy.Url.Open(url);
			}
			await update();
		};
  }
  onMount(() => {
    // Create the LemonSqueezy global object
		window.createLemonSqueezy();
		LemonSqueezy.Setup({
			eventHandler: ({ event, data }) => {
        // Listen for events, if payment was successfull 
        // submit the data 
				if (event === 'Checkout.Success') {
					console.log('Will submit the form');
					// Since payment was made, send the audio to the backend
					// append the order id to allow the BE to check the paid status
					orderId = data.order.data.id;
					LemonSqueezy.Url.Close();
					paidForm.submit();
				}
			}
		});
	});

</script>

<form
  method="POST"
  action="/?/prepayment"
  use:enhance={onGetPayment}
>
  <button type="submit">Pay</button>
</form>

Awesome! Thanks @matiasfha 🙌