If you do not have a subsidiary/branch in Italy that can apply for intra-company permits, the only option would be as assignment pursuant to the service agreement that you have executed with the Italian customer (under art. 27(i) Immigration Law). The application for obtaining the work permit must be filed by the Italian company but the workers remain hired by the foreign company. These permits are for a maximum duration of 4 years (2 + 2 years). A posted worker notification under EU Directive 67/2014 is required in this case.
