[NTG-context] Commands with arguments with MetaPost

I would like to be able to do something like this \def\foo#1{ \startMPcode fill (0,0)--(598,0)--(598,50)--(0,50)--cycle withcolor green; label.rt(btex #1 etex),(50,25); \stopMPcode } and then use this in my ConTeXt document by \foo{Some text} However, my syntax seems to be incorrect, and I would appreciate any help in fixing it. Thanks, Troy Henderson