Not everything needs to be stated for the audience to know it happened.
It's a given that people voted for Tobirama, considering that if Hashirama wasn't allowed to give Madara the title by just his say, then he wasn't allowed to give the title to his brother just through his say either. The only reason Tobirama had the authority to give the title to Hiruzen through his say was because he was about to die and the Hokages need to be replaced as soon as possible. They were in the middle of a war and far from Konoha- not an opportune time for elections.
Not that it matters in the least whether or not people voted for him. People sided with the Senju, even the Uchiha sided with the Senju over their own leader who just wanted the best for them, Madara. And, of course everyone else was content with the Senju bros ruling as well. The general village population favored both of them over people like Madara. There's also the case of Tobirama being the one to advance Konoha the most with the laws and regulations he set in place. Due to him, Konoha became the strongest country. In the databooks, it was also stated that he was positively received and viewed as someone who was "courageous". Plus, nothing considerably bad happened during his reign (no rebellions, nothing like that).