  We present the first ever measurements of femtoscopic correlations between
the K$^0_{\rm S}$ and K$^{\rm \pm}$ particles. The analysis was performed on
the data from Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV measured by the
ALICE experiment. The observed femtoscopic correlations are consistent with
final-state interactions proceeding via the $a_0(980)$ resonance. The extracted
kaon source radius and correlation strength parameters for K$^0_{\rm S}$K$^{\rm
-}$ are found to be equal within the experimental uncertainties to those for
K$^0_{\rm S}$K$^{\rm +}$. Comparing the results of the present study with those
from published identical-kaon femtoscopic studies by ALICE, mass and coupling
parameters for the $a_0$ resonance are constrained. Our results are also
compatible with the interpretation of the $a_0$ having a tetraquark structure
over that of a diquark.
