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Using the instructions supplied by Squarespace, you will be tempted to include the A records in the Subdomain section. WRONG! If you do this, your settings will be rejected and you will get an error message complaining about the @ sign.

What worked for me:

Under the Domain Records area:
  Do NOT include a Forward instruction - even though Dynadot says to
    (Maybe there's a good reason for the Forward, but the Squarespace person I talked to didn't seem to think so.)
  DO define four A records using @ signs and the IP addresses supplied by Squarespace
    (They are 198.185.159.144, 198.185.159.145, 198.49.23.144, 198.49.23.145)

Under the Subdomain area:
  Define the first CNAME using the unique code supplied by Squarespace
  Define the second CNAME using ext-cust.squarespace.com (NOT ext.squarespace.com like Dynadot says)

https://www.loom.com/share/a8e9e2018988448b9d6eaaf1808dde34

This does not work.

Thanks for the update, we'll look into this.