\setupinteraction[state=start]

\setupforms[export=yes] % create an xfdf file too

\setupfield[whatever][height=1cm,width=10cm]

\definefield[testa][text][whatever][zomaar wat]
\definefield[testb][text][whatever][wat anders]

\startTEXpage[offset=1cm]

    \field[testa]

    \blank

    \field[testb]

\stopTEXpage
