  A measurement of the ratio of branching fractions $R$(J/$\psi$) =
$\mathcal{B}$(B$^+_\text{c}$ $\to$
J/$\psi$$\tau^+\nu_\tau$)/$\mathcal{B}$(B$^+_\text{c}$ $\to$
J/$\psi$$\mu^+\nu_\mu$) in the J/$\psi$ $\to$ $\mu^+\mu^-$, $\tau^+$ $\to$
$\mu^+\mu_\tau\overline{\nu}_\tau$ decay channel is presented. This measurement
uses a sample of proton-proton collision data collected at a center-of-mass
energy of 13 TeV by the CMS experiment in 2018, corresponding to an integrated
luminosity of 59.7 fb$^{-1}$. The measured ratio, $R$(J/$\psi$) = 0.17$^{+
0.18}_{- 0.17}$ (stat) $^{+ 0.21}_{- 0.22}$ (syst) $^{+ 0.19}_{- 0.18}$ (theo)
= 0.17 $\pm$ 0.33, agrees with the value of 0.2582 $\pm$ 0.0038 predicted by
the standard model, which assumes lepton flavor universality.
