Hello,
The idea is to make a chapter header as with the "titlesec" package. My code compiles under texlive context 2018 but does not compile with last beta contest standalone.
Thank you

Fabrice

############################################

% macros=mkvi

\setuppagenumbering[location=footer]

\startusableMPgraphic{NumberHead}
     draw outlinetext.f
         ("\bf\namedheadnumber{chapter}")
         (withcolor "lightgray")
         ysized 50pt ;
\stopusableMPgraphic

\unexpanded\def\processMPheadnumber#1%
{\useMPgraphic{NumberHead}}

\setuphead
   [chapter]
   [command=\HeadTitle,
    headstyle=\ss,
    numbercommand=\processMPheadnumber]

\unexpanded\def\HeadTitle#1#2%
{\framed
    [frame=off,
    bottomframe=on,
    width=broad,
    align={broad,nothyphenated,left}]
    {#1\blank[nowhite]#2}}

\starttext

\dorecurse{6}{\startchapter
[title={Fist chapter}]
\input knuth
\stopchapter}

\stoptext

########################################