Probably the UV spectrum of HP-b-CD possesses a Cross Section sufficiently large below 400 nm. If you have a UV-visible equipment to measure in that region you can get the relative amount of HP-b-CD by integrating the absorption band.
However, the only way to know the actual concentration of your sample is to perform a calibration curve (concentration vs absorbance). With it you will be able to obtain the real concentration of your compound from any absorption spectrum.