Covariant in two way repeated measures in SPSS

Hi everyone, I'm a total noob in SPSS. I typically use graphpad prism and was using mixed effect analysis to analyze the effect of a supplement given over time using a cross over design on an inflammatory marker. I then realize the BMI of our volunteers could influence the inflammatory markers and would like to control for this factor. Does anyone know how to include a covariant in a two way repeated measures ANOVA in SPSS? Or in graphpad prism? Thanks a lot, in advance!