Hi, I got around this by looking at the network request sent when creating a redirect for a random subdomain, and manually sending the request with a modified JSON body with cURL. See this example:
curl 'https://account.squarespace.com/api/account/1/domains/xxxxxxxxxxxxxxxxx/forwarding-presets' \
-H 'accept: application/json, text/plain, */*' \
-H 'accept-language: en-US,en;q=0.9' \
-H 'content-type: application/json' \
-H 'cookie: xxxxxxxxxxxxxxxxxxxxxxxxxxx' \
-H 'origin: https://account.squarespace.com' \
-H 'referer: https://account.squarespace.com/domains/managed/xxxxxxxxxxxxxx/website' \
-H 'x-csrf-token: xxxxxxxxxxxxxxxxxxxxxxxxxxxxx' \
--data-raw '[{"templateId":"SQUARESPACE_SUBDOMAIN_FORWARDING","subdomain":"www","forwardingUrl":"http://your_url_here.com","temporaryRedirect":false,"pathForwardingEnabled":true,"sslEnabled":true}]'