\definebar[MyBar][color=green,rulethickness=2.8,order=background,offset=1.5,continue=yes]

\starttext
  1 + 2 = \startbar[MyBar]3~kN\stopbar

  $1 + 2 = \startbar[MyBar]3~\text{kN}\stopbar$

  $1 + 2 = \inframed[frame=off,background=color,backgroundcolor=green,offset=0pt,]{3~\text{kN}}$
\stoptext