First, create a Squarespace Scratch Card plugin on Common Ninja.
On the plugins catalog, find the Scratch Card and enter the Editor.
When you're done creating the Scratch Card plugin, click on the Save Changes button at the right top corner of the editor.
If you don't have an account on Common Ninja, you'll have to register first.
When you're ready to add the Scratch Card plugin to your Squarespace store, click the Add To Website button at the top bar of the Editor.
On the window that opens, click the Copy Icon to copy the plugin embed code. In the next steps, you will paste this code into your Squarespace store.
Now, let's head to your Squarespace website editor.
From your Squarespace dashboard, go to the blog posts you want to edit or add a new one.
On the page, click on Add Block wherever you wish to add the Scratch Card.
Now, hit the Embed option from a new popup dialog.
Move the Embed block and place it where you want the Scratch Card to show.
Click on the Edit button at the top of the block.
Now, in the Block's content window, simply paste the Scratch Card embed code you've copied before,
Click on the page everywhere outside of the content window to close it.
Please note: the Scratch Card will show only on your published site.