The orbit to which electrons will first be filled should have less energy than another. This is Klechkovsky's rule. The energy level is determined by the following formula: n + l. Therefore, if the energy level of the orbital is less than another, then this orbital will be filled first. For 5d orbitals, the energy level is 5 + 2 = 7. For 6s orbitals the energy level is 6 + 0 = 6. Therefore, the orbital will be filled with 6s first.
Therefore, the orbital will be filled with 6s first.