For 2019 tax returns the official FTA exchange rates are:

    IncomeFortune
USD1   =  CHF 0.99365558CHF 0.968374
EUR1   =  CHF 1.11247283CHF 1.087
GBP1   =  CHF 1.26829753CHF 1.282822

Foreign exchange rates are available here.

Average monthly exchange rates are available herei.

Daily exchange rates supplied by the customs service are available here.