To create a hotkey for the log function on the TI-89, users can utilize the change of base formula, which allows for calculations involving logarithms of different bases. Since there isn't a dedicated key for base 10 logs, the formula is expressed as c = ln(b)/ln(a). For practical use, programming a hotkey may not be effective, as it typically displays results in the program window rather than the main window. Instead, defining a user-defined function is recommended. This can be done by creating a function like Logg(a,b) = ln(b)/ln(a), which allows for more straightforward calculations. Additionally, users can explore moving the log function to a custom menu for easier access.