When you use "header=high" you can't have a header because the block for it no longer exists, compare the results for "header=high" and "header=empty" in the following example.

%%%% begin example
\showframe

\starttext

\setuphead[chapter][header=high]

\chapter{Lorem Ipsum}

\dorecurse{10}{\samplefile{lorem}}

\setuphead[chapter][header=empty]

\chapter{Lorem Ipsum}

\dorecurse{10}{\samplefile{lorem}}

\stoptext
%%%% end example

As you have guessed \definetext is the way to create a custom header for the first page of your \chapter, the command has like \setupheadertexts a variable number of arguments.

The third argument with the horizontal position can only be used when you use two or four arguments to set texts on the left and right side but isn't in my example because a) the text position is the default and b) I pass only one argument for the content because I wan't the text in the middle.

%%%% begin example
\showframe

\definetext
  [chapterheader] % identifier
  [header]        % vertical position (header/footer)
% [text]          % horizontal position (text/margin)
  [This is a custom header for \tex{chapter}] % content

\setuphead[chapter][header=chapterheader]

\starttext

\chapter{Lorem Ipsum}

\dorecurse{10}{\samplefile{lorem}}

\stoptext