§1. A person aspiring to the diaconate or presbyterate is not to be ordained unless he has first been enrolled among the candidates through the liturgical rite of admission by the authority mentioned in cann. 1016 and 1019; his petition is previously to have been written in his own hand, signed, and accepted in writing by the same authority.
§2. A person who has been received into a clerical institute through vows is not bound to obtain this admission.
Source: Code of Canon Law (Vatican.va)