There was no absentee ballot. If you were a Roman citizen living in Tarraco and wanted to vote in the Roman assembly in the election of a magistrate or the passing of a law, then you needed to go to Rome to do so.

During the early imperial period, it became less important anyway. The magistrates were not really voted into office by the people following the reign of Tiberius (they were just appointed by the Emperor), and the passing of laws in the assembly died off by the end of the 1st century CE. Thus by 100 CE, you had nothing really important to vote on anyway.