@Christoph-Hart Okay great, this fix already allowed me to pass the plain string into the function.
Another problem, that also was mentioned in the referenced thread is that data is not being processed as text/plain or plain JSON as it looks like HISE is sending the data as URL-encoded form data (like a web form submission)
This leads to problems in calling the sign-in API endpoint of Moonbase - this is how Moonbase expects to receive sign-in data:
POST https://demo.moonbase.sh/api/customer/identity/sign-in?email=test@example.com
Content-Type: text/plain
Password1234!
I need to embed the mail into the URL endpoint and pass on the password as plain text. This is how I implemented it within HISE:
makeHttpRequest("POST", "/customer/identity/sign-in?email=" + email, password, function(success, response)
Due to the faulty URL encoding the data is being received in the format shown below:

I am not sure how to create a workaround for this..