Once you have created your SquareSpace domain in your MailMunch account, follow the few easy steps mentioned below.
Step 1: Sign in to your MailMunch account, and choose your site.
Step 2: Click on the Site Settings tab present on bottom left of the screen.
Step 3: Click on the GET SITE CODE button on the top right corner of the screen.
Step 4: Copy the site code.
Step 5: Login to your SquareSpace account.
Step 6: Navigate to the Home Menu, and click on the Settings--->Advanced--->Code Injection options.
Step 7: Locate the Header, and paste the Site Code that you copied earlier from your MailMunch account.
Step 8: The code pasted here will be injected into the <head></head> tag of all the pages of your SquareSpace webpage.
Step 9: Click on the Save button, and you are done! Any forms that you have created in your MailMunch account should now be active on your SquareSpace page.
For further assistance, check out the resource on adding a code through Code Injection by SquareSpace.