It is not currently possible to adjust a client's personal allowance in Voyant to reflect the marriage allowance. The amount is somewhat marginal as it only allows for the transfer of up to £1,260 (2022/23) unused allowance to a spouse, so we haven't coded for it.
We recommend that you enter any tax saved as a result of this allowance as 'Other Income', ensuring that you select 'non-taxable'.