According to the manual, it's K:MIXTURE1_SET 0-100 or L:BKSQ_MixtureLeverPosition_1 0-1. These must not have worked for me I guess, or I wouldn't be using the one I indicated above.

There's also a section in the FAQ (from the manual) that addresses the Turbocharged mixture:

Why does the mixture behave strangely in the turbocharged
version, and I cannot bind it to hardware controls?
Microsoft Flight Simulator’s turbocharger simulation has been significantly flawed for several
generations. This aircraft has a custom turbocharger that fixes nearly all of these issues, and is
much more realistic, as a result. To make these changes, the new “Input Event” system is used
to intercept hardware and key-bindings for the mixture control axis. Please make sure that your
hardware bindings are using the Key Events, such as “K:MIXTURE1_DECR_SMALL”, or
“K:MIXTURE1_SET” to set the mixture, and NOT setting either “A:GENERAL ENG MIXTURE
LEVER POSITION:1”, or “B:FUEL_Mixture_1_Set”. Alternatively, setting
“L:BKSQ_MixtureLeverPosition_1” from 0-100 will also work to set the mixture axis.