Have you looked in the ABAP dump analysis (TA: ST22)? Here you may see a more detailed error message. Depending on the screenshots it is very difficult to determine the cause/solution.
Maybe your question is "what is XML" or you don't know how to locate "malformed XML", then paste the XML code in the forum. It may also be due to unexpected character encoding, which makes SAP read incorrectly the XML code.
Fix your HTTP request in Postman. I used it and it's very powerful, there's no good reason to say that it doesn't work with Postman (I mean, without giving any argument).
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