Use functions in a configuration in the same places you use a string literal. You can use examples in this topic as guidelines.
 <Label  text="{$_i18n.from}"> 
 <Label text="{$_i18n.since} {$_format.dateTime($_now(-24),short)}"> 
<Title tileID="leave_request_list” title="{$_i18n.leaverequests}">
 <p pid=”imageSrc” value="lr_tab_icon"/>
For further examples, see the extensibility samples.