For an Custom Action Button:
If you want a button that performs a custom action (without submitting the form), use type="button":
<Field name="customButton" displayName="Click Me" type="button">
<Attributes>
<Map>
<entry key="buttonLabel">
<value>Click Me</value>
</entry>
<entry key="onClick">
<value>
<Script>
<Source>
// Your custom logic here
log.debug("Button clicked!");
</Source>
</Script>
</value>
</entry>
</Map>
</Attributes>
</Field>