If you just want to implement this generically, it shouldn't be hard to build up from the mathematical definition.
- Divide Vin by Vref with a Divide block or a Gain block. The block output is Vin/Vref.
- Take the log of the output with a Log block. The block output is ln(Vin/Vref).
- Multiply by K with a Gain block. The block output is K*ln(Vin/Vref).
If you want that to look pretty, you can package it in a subsystem, and perhaps create a mask to define K and Vref.
