  We study the fusion processes $W^-W^+\to t\bar t$ and $ZZ\to t\bar t$
observable at a future $e^-e^+$ collider and we discuss their sensitivity to an
$Htt$ form factor which may be due to compositeness, in particular when the $H$
and the top quark have common constituents. We make an amplitude analysis and
illustrate which helicity amplitudes and cross sections for specific final
$t\bar t$ polarizations are especially sensitive to this form factor.
