All field keywords beginning with html_form_ are passed directly into the form hash. Thus, they are really defined in form.tt which ships with Gantry.
html_form_
html_form_onchange sets Javascript to execute when a field of type select changes. Usually this is something like:
html_form_onchange
html_form_onchange `some_function()`;
It's up to you to make sure the wrapper or template defines the Javascript function.
To see an example, build:
bigtop -c example.bigtop all
Change to the newly created Kids directory and look in the form method in lib/Kids/GEN/Child.pm.
To install Bigtop, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Bigtop
CPAN shell
perl -MCPAN -e shell install Bigtop
For more information on module installation, please visit the detailed CPAN module installation guide.