If you want to include a Pinterest button into your Landing Page, first you'll need to go here https://business.pinterest.com/en/widget-builder.
Once there you'll see that you can create the "Pin it" button, the "Follow" button and three different formats of the "Pinterest Widget". Once you finish to create the one you want click on the "Build it!" button and you'll see the code you need to copy.
Now get into the Landing Page where you want to include it, go to the advanced tools and select the HTML element. Now just paste the code you got before and that's all.
Remember that on the canvas of your Landing Page you'll see a message saying "This is a placeholder the actual content is visible in the preview or published page".
So as the message says, you'll see the button on the preview of your Landing Page or once you publish it.