Traveling-wave interaction between flowing plasma and electromagnetic wave supported by external slow wave structure is analyzed in the collisionless regime. The plasma is described with the collisionless Boltzmann equation and the interaction is analyzed using the coupled-mode approach. The analysis is applied to a plasma consisting of drifiting charge carriers in a solid. Illustrative examples demonstrate interaction leading to wave growth for frequencies in the far-infrared regime. It is suggested that experimental measurement of wave growth or damping due to this effect can provide information about the velocity distribution of drifting carriers in the solid.