Once you have created your SquareSpace domain in your MailMunch account, click on "Setting Gear" inside that domain and then click on Get Site Code which should display the site code for that domain.Copy this code.
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1. Login to your SquareSpace account
2. In the Home Menu click Settings--->Advanced--->Code Injection.
3. Locate Header and paste the MailMunch site code you copied earlier from your MailMunch account
4. The code pasted here will be injected into the <head></head> tag of all the pages of your SquareSpace webpage.
5. Click Save and you are done. Any forms you have created in your MailMunch account should now be active on your SquareSpace page.
For more assistance with this you can also reference SquareSpace support article for adding a code through Code Injection.