FullTextEntityManager fullTextEntityManager = org.hibernate.search.jpa.Search.getFullTextEntityManager(em);

List<String> fieldNames = indexedFields.stream().map(IndexedField::getName).collect(Collectors.toList());
String[] fieldArray = new String[fieldNames.size()];
fieldNames.toArray(fieldArray);
Map<String, Float> boostValues = indexedFields.stream().collect(Collectors.toMap(IndexedField::getName, IndexedField::getBoost));
MultiFieldQueryParser standardParser = new MultiFieldQueryParser(fieldArray, analyzer, boostValues);

Query query = standardParser.parse(queryString);

FullTextQuery ftq = fullTextEntityManager.createFullTextQuery(query, IndexedEntity.class);