A search for a charged Higgs boson decaying into a heavy neutral Higgs boson and a boson is presented. The analysis targets the boson decay into a pair of tau leptons with at least one of them decaying hadronically and with an additional electron or muon present in the event. The search is based on proton-proton collision data recorded by the CMS experiment during 20162018 at \sqrt{s} = 13~\mathrm{TeV}, corresponding to an integrated luminosity of 138~\mathrm{fb}^{-1}. The observed data are consistent with standard model expectations. Upper limits at confidence level are set on the product of the cross section and branching fraction for an in the mass range of 300 to 700~\mathrm{GeV}, assuming an with a mass of 200~\mathrm{GeV}. The observed limit ranges from 0.080~\mathrm{pb} at 300~\mathrm{GeV} to 0.013~\mathrm{pb} at 700~\mathrm{GeV}. These are the first limits on this process at the LHC.