I have found this answer
And I transformed that to Javascript in Mirth
var schemaFile = new Packages.java.io.File("C:\\schema.xsd");
var url = new Packages.java.net.URL("file:C:\\input.xml");
var xmlFile = new Packages.javax.xml.transform.stream.StreamSource(url);
var schemaFactory = Packages.javax.xml.validation.SchemaFactory.newInstance("http://www.w3.org/2001/XMLSchema");
var schema = schemaFactory.newSchema(schemaFile);
var validator = schema.newValidator();
try {
validator.validate(xmlFile);
logger.info('valid');
} catch (err) {
logger.error(err.toString());
}
You need to download JAXP (Java API for XML Processing) from here
I hope it helps others