A preliminary result of a new measurement of the $K^+ \to \pi^+ \mu^+ \mu^-$ form factor parameters and branching fraction using the NA62 Run $1$ data set collected in the years $2016 - 2018$ is presented. From the observed sample of $28011$ $K^+ \to \pi^+ \mu^+ \mu^-$ event candidates with negligible background contamination, the form factor parameters $a_+ = -0.592 \pm 0.013_\text{stat} \pm 0.007_\text{syst} \pm 0.001_\text{ext}$ and $b_+ = -0.699 \pm 0.046_\text{stat} \pm 0.035_\text{syst} \pm 0.003_\text{ext}$, and the branching fraction $\mathcal{B}(K^+ \to \pi^+ \mu^+ \mu^-) = (9.27 \pm 0.07_\text{stat} \pm 0.08_\text{syst} \pm 0.04_\text{ext})\times 10^{-8}$ are obtained.