If you want to use embed forms for upsell products, you must disable the 1-click upsell feature for the campaign: https://support.paykickstart.com/articles/0405495-setting-up-1-click-upsells

Step 1:

Click on the Campaigns at the left navigation menu.

Click on the Edit icon of the funnel you want to use the embed form with.

Locate the product you want to use the embed form with and click on Edit.

Step 2:

Now, click on Checkout Options and enable Embed form.

Step 3:

Choose your embed design template (Embed from Step 1)

Step 4:

Choose your embed form design options (Embed for step 2)

Note: Design options will vary depending on the selected template

Step 5:

Finally, copy that Embed Code and place it on your own domain’s checkout page. (Embed from step 3)

Step 6:

Save the main funnel settings and exit the funnel.