[utw-Mephisto]

You only need to change one template to make it default ticked. There is no need to modify any files !!!

To do so, open the template pm_newpm

Search for

<if condition="$show['trackpm']"><div><label for="cb_receipt"><input type="checkbox" name="receipt" value="1" id="cb_receipt" $checked[receipt] />$vbphrase[request_receipt_for_message]</label></div></if>

and change to

<if condition="$show['trackpm']"><div><label for="cb_receipt"><input type="checkbox" name="receipt" value="1" id="cb_receipt" checked="checked" />$vbphrase[request_receipt_for_message]</label></div></if>

Or, if you would like to give them an option, use a IF condition in the template ..