How to extend existing FlexForm select options of a TYPO3 plugin using Page TSconfig

Sometimes existing options of a TYPO3 plugin may not fully suite the project requirements. As an example, I refer to my TYPO3 extension "Event Management and Registration" (sf_event_mgt). The extension allows to select the ordering of records by a specific field in the FlexForm plugin options as shown on the screenshot below.

The 3 options shown are configured in the Flexform options for the field "settings.orderField".

In a project it was required to order by a custom field which was not part of the main extension. So I added the custom field named "slot" to the extension using an extending extension for sf_event_mgt.

In order to allow the new field as sorting field, the field "slot" needs to be added to the allowed ordering fields using TypoScript (note, this step is only specific to the extension sf_event_mgt).

plugin.tx_sfeventmgt {
  settings {
    orderFieldAllowed = uid,title,startdate,enddate,slot
  }
}

Finally the FlexForm of the plugin needs to be extended, so the new field appears in the "Sort by" select field. In order to do so, the following Page TSconfig has been added:

TCEFORM.tt_content.pi_flexform.sfeventmgt_pievent.sDEF.settings\.orderField {
  addItems.slot = slot
  altLabels.slot = Slot
}

You might notice the backslash before the dot in "settings\.orderField". This is required to escape the dot of the fieldname "settings.orderField", since Page TSconfig also uses dots to separate between configuration options/levels.

After adding the Page TSconfig, the plugin now shows the new field.

Pretty cool and not a single line of PHP code required :-) 

Reference: TYPO3 TCEFORM