  Low-temperature variable-energy electron irradiation was used to induce
non-magnetic disorder in a single crystal of hole-doped iron-based
superconductor, Ba$_{1-x}$K$_x$Fe$_2$As$_2$, $x=$0.80. To avoid systematic
errors, the beam energy was adjusted non-consequently for five values between
1.0 and 2.5 MeV, whence sample resistance was measured in-situ at 22 K. For all
energies, the resistivity raises linearly with the irradiation fluence
suggesting the creation of uncorrelated dilute point-like disorder (confirmed
by simulations). The rate of the resistivity increase peaks at energies below
1.5 MeV. Comparison with calculated partial cross-sections points to the
predominant creation of defects in the iron sublattice. Simultaneously,
superconducting $T_c$, measured separately between the irradiation runs, is
monotonically suppressed as expected since it depends on the total scattering
rate, hence total cross-section, which is a monotonically increasing function
of energy. Our work confirms experimentally an often-made assumption of the
dominant role of the iron sub-lattice in iron-based superconductors.
