In the technical documentation, I think there is an error at this page https://docs.axelor.com/adk/5.1/dev-guide/modules/coding.html
this part :
<form name="contact-form" model="com.axelor.contact.db.Contact">
...
<field name="email" onChange="com.axelor.contact.db.Contact:validate(email)"/>
...
</form>
Must be :
<form name="contact-form" model="com.axelor.contact.db.Contact">
...
<field name="email" onChange="com.axelor.contact.web.HelloController:validate(email)"/>
...
</form>