> Marketing Cloud: to retrieve the list of data extension within a folder:

This worked for me, tested via Postman and even with the Python FuelSDK
```
<?xml version="1.0" encoding="UTF-8"?>
<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope" xmlns:a="http://schemas.xmlsoap.org/ws/2004/08/addressing" xmlns:u="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
    <s:Header>
        <a:Action s:mustUnderstand="1">Retrieve</a:Action>
        <a:To s:mustUnderstand="1">https://{{et_subdomain}}.soap.marketingcloudapis.com/Service.asmx</a:To>
        <fueloauth xmlns="http://exacttarget.com">{{dne_etAccessToken}}</fueloauth>
    </s:Header>
    <s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
        <RetrieveRequestMsg xmlns="http://exacttarget.com/wsdl/partnerAPI">
            <RetrieveRequest>
                <ObjectType>DataExtension</ObjectType>
                <Properties>ObjectID</Properties>
                <Properties>CustomerKey</Properties>
                <Properties>CategoryID</Properties>
                <Properties>Name</Properties>
                <Properties>IsSendable</Properties>
                <Properties>SendableSubscriberField.Name</Properties>
                <Filter xsi:type="SimpleFilterPart">
                    <Property>CategoryID</Property>
                    <SimpleOperator>equals</SimpleOperator>
                    <Value>[CategoryIDvalue]</Value>
                </Filter>
            </RetrieveRequest>
        </RetrieveRequestMsg>
    </s:Body>
</s:Envelope>
```