From the Jewish Press:
[inserting his name], son of
But this Gemara seems to be talking about a remedy for all Jews entering a city, many of whom obviously do not descend from Joseph!
Some argue that, indeed, the suggested remedy is effective only for those who turn out to be descendants of Joseph.
Rashi and Metzudat David explain that since Joseph sustained his brothers and their families in Egypt, they are referred to by his name.”