This can be achieved by utilising the “Manual fund entry” within the Add model portfolio screen and creating a manual fund that is 100% allocated to Money Markets. If importing the funds for the Model Portfolio, do this first before adding the cash element.
The Money Market asset that is available within Manual fund entry is described as “Modelled to simulate a portfolio of UK cash and equivalents with characteristics and average terms typical of money market funds". An example would be the return on 91-day UK Treasury bills.