I'm not sure. I was connecting it to the I2C pins on the GPIO headers on the side of the Portenta H7 (D12 and D11). According to the Portenta schematic, that's on the Form Factor MKR block (page 8), which goes to I2C3. From there, I'm not sure.
Yes, that would work if I powered everything from 3.3 volts (hooking up to the 3.3 volt pin on the Portenta and the magnetometer). But, on a board that I routed, I used 5 volts instead of 3.3 volts, and my external pull-up resistors (not on the magnetometer) went from SCL/SDA to +5 volts. This was something that I'm trying to examine after the fact, in order to see what I shouldn't do in the future.