How are loading times to be proven with this tachograph prior to departure?

You answered correctly. The loading times must be entered directly into the tachograph before departure. This is a legal requirement to ensure that all work and rest periods are accurately recorded and can be checked later during inspections. Making a note on a separate sheet or only telling the employer is not enough—the tachograph is the official record. For future reference, always use the tachograph to document these periods when required by law.