If you are looking for an quick & easy way to create XSD (schema) from Dynamics 365 for finance and operations. Below is the way,
Select the entity for which the XSD must be created and choose
target data format as ‘XML-Element’
Export the data.
Download the XML file.
Open the file in Visual Studio and click XML -> Create Schema

The XSD file will be generated, which can be shared with 3rd
party applications.