This is not possible at present. As needed withdrawals (those taken automatically by the software to fulfill expenses) will be taken as UFPLS withdrawals, thereby comprising a combination of tax-free and taxable income. There is no mechanism by which the software is able to, automatically, take withdrawals in the form of tax-free cash only.
The option exists to schedule, manually, 'planned withdrawals' of tax-free cash (only) from a money purchase account, using Flexi-Access Drawdown. You will find this option on the 'Planned Withdrawals' screen. If you schedule a 'Planned Withdrawal' and set the 'Pension Withdrawal Strategy' field to 'Flexi', as illustrated below, the software will (1) interpret the stated 'amount' as referring to the desired PCLS and (2) will crystallise sufficient funds to realise the desired PCLS. The balance (taxable funds), will be, automatically, deposited to the 'linked' pension drawdown account.
To emphasise, once more, this Flexi-Access Drawdown (FAD) option exists only for planned withdrawals - i.e., withdrawals that have been scheduled, manually - and not for withdrawals taken, automatically, by the software, on an 'as needed' basis.