An extensive 3D numerical simulation of a transient electromagnetic field evolution inside large ELI-Beamlines structures will be presented. Laser-plasma generated Electro-Magnetic-Pulse (LP EPM) propagation in an experimental area of ELI L4 beamline, namely inside vacuum vessels of compressor, large-size beam-transport manifold and P3 interaction chamber was successfully simulated. High frequency RF pulse propagation, confinement and attenuation was investigated in order to reduce hazardous EMP effects to laboratory infrastructure. Entire size and complexity of L4 beamline chain requires a full-3D time-dependent calculation with large demands on computing hardware performance and memory. Dedicated multi-processor, multi-core server was used for large data import, calculation, output processing and field visualization with ANSYS AEDT HFSS module using Transient Solver. Simulations confirm that use of common construction materials for beamline-vacuum-vessels does not attenuate EMP sufficiently, and the structure behaves as a large-size EMP reverberation chamber with a long decay time. To mitigate detrimental EMP effects, an efficient RF/microwave blocking and absorption strategies are discussed. Electronic industry ferrimagnetic ceramics were examined for a stable, vacuum compatible, clean-room compatible, nuclear activation compatible RF/microwave absorbing material in MHz/GHz region. Materials economically viable in large quantities required for large structures were identified. Initial tests of selected materials were performed and an absorbing structure optimization is in progress.