Shopify Forms can be added using the App Island feature, which works on all section layouts or pages with Shopify Markets enabled.
Open the Insert Panel
Go to Insert → Elements → Embed.
Select App Island.
Drag and Drop the App Island Element
Place the App Island element into your section.
Publish Your Section
Click Publish to make your section live.
Open the Shopify Theme Customizer
In Shopify, open the Theme Customizer.
Add the Section to Your Page
Add the section you created in Instant to your Shopify page.
Insert the Shopify Forms Block
Under the selected section, click Add block.
Choose the Shopify Forms app block.