Using Mailerlite with Squarespace (constant upgrade to do basic things)

I have a squarespace business website. I also use mailerlite new version. Which cannot be directly integrated with squarespace. I've tried to use a zap to connect the two, but the format of my form is glitching. I suspect because my developer api key is (read only). To get it to do anything else, I have to upgrade to a premium.  The zap works internally with a zap test, but a test through squarespace doesn't. Has anyone else run across this problem? Do I have to change my newsletter format to classic mailer lite to get it to work? And if so, what happens when all of mailer lite switches over to the new version? the help pages on squarespace is a misnomer as it's very little "help" at all. Tons of jargon to sift through and nothing specific to what I need. No one to call. and no chat line available.