I have checked with mailerlite below is their response. I looks Apiway needs to update things at their end.
"It looks like Apiway needs to fix it on their end as this integration doesn't work with the New MailerLite that you're using. You can reach out to their support to let them know that now we have 2 versions of MailerLite - MailerLite Classic and New MailerLite.
You're using New MailerLite now."
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