The method of a variational wavefunction has been used to study the effect of the electron-LO-phonon interaction on the binding energy of the ground bound state of an isolated hydrogenic impurity in a GaAs-Ga_1-xAl_xAs quantum well subjected to an external electric field. The binding energy is obtained as a function of the impurity position z_i, the well width L and the electric field strength F. It was found that the polaronic correction to the impurity binding energy is quite significant. The correction ranges from about 8% when the impurity is placed at the centre to 13-15% when the impurity is placed at the boundary for the thin quantum well.