Ok, \starttexdefinition allows blanks, apparently just like setups.
But, it still fails exactly like the version with \def


\setuphead[chapter][
    before={Before: \structureuservariable{author}\blank},
    beforesection={Beforesection: \structureuservariable{author}\blank},
    insidesection={Insidesection: \structureuservariable{author}\blank},
    after={After: \structureuservariable{author}\blank},
    ]

\def\myChapterCommand#1#2{\structureuservariable{author}\space\blank}
\define[2]\myChapterCommand{\structureuservariable{author}\blank #1\space#2}
%\def\myChapterCommand#1#2{\structureuservariable{author}\blank #1\space#2} % does not work

% \starttexdefinition myChapterCommand #1 #2 % doesn't work either, but at least blanks should work
%   \structureuservariable{author}
%   Test
%   \blank
%   #1\space#2
% \stoptexdefinition


\starttext

\myChapterCommand{1}{asdf}

\startchapter[title=First][author=First Author]
\input knuth
\stopchapter

\startchapter[title=Second][author=Second Author]
\input knuth
\stopchapter

\setuphead[chapter]
    [
    before=,after=,beforesection=,insidesection=,
    command=\myChapterCommand,
    ]

\startchapter[title=Third][author={Third Author}]
\input knuth
\stopchapter


\stoptext

Von: denis.maier@unibe.ch <denis.maier@unibe.ch>
Gesendet: Dienstag, 12. September 2023 17:48
An: ntg-context@ntg.nl
Betreff: [NTG-context] Using structureuservariables before the heading

Hi,

is it possible to access a structureuservariable before the heading's title ?
Using the <before> key does not seem to work.

As an aside, I've tried to use the < command > key to provide my own command, but while \define works here (but ignores \blank), \def gives me an error.
Why is that?
How would you add blanks to a user-defined command?

Best,
Denis



\setuphead[chapter][
    before={Before: \structureuservariable{author}\blank},
    beforesection={Beforesection: \structureuservariable{author}\blank},
    insidesection={Insidesection: \structureuservariable{author}\blank},
    after={After: \structureuservariable{author}\blank},
    ]

\def\myChapterCommand#1#2{\structureuservariable{author}\space\blank}
\define[2]\myChapterCommand{\structureuservariable{author}\blank #1\space#2}
%\def\myChapterCommand#1#2{\structureuservariable{author}\blank #1\space#2} % does not work

\starttext
\macro{1}{2}

\startchapter[title=First][author=First Author]
\input knuth
\stopchapter

\startchapter[title=Second][author=Second Author]
\input knuth
\stopchapter

\setuphead[chapter]
    [
    before=,after=,beforesection=,insidesection=,
    command=\myChapterCommand,
    ]

\startchapter[title=Third][author=Third Author]
\input knuth
\stopchapter


\stoptext