To couple energy into and out of a waveguide use:

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To get energy into or out of a waveguide you need a device that can transfer the energy between the guided microwave field and another medium. A thin piece of wire placed in or near the waveguide acts as a small antenna or probe, picking up and injecting a portion of the wave’s field so energy can flow between the waveguide and the external circuit or radiator. This coupling element is designed to be small compared to the wavelength and is adjustable to control how much energy is transferred without heavily disturbing the waveguide mode. The other options aren’t suitable for this direct, efficient transfer: a wide copper sheet would disrupt or short the field, an LC circuit is a lumped-network component not a direct coupler, and capacitive coupling isn’t the standard, practical method used for entering and exiting waveguide energy in typical setups.