TYPO3 Core

EDIT:

in Form/FormDataProvider/TcaFlexProcess there is a comment on line 366ff saying this:

// It is possible to have a flex field field with of foreign_table (eg. type=select) that has markers in
// a foreign_table_where like ###PAGE_TSCONFIG_ID###. It was possible to set this in page TSConfig for flex fields like this:
// TCEFORM.theTable.theFlexfield.PAGE_TSCONFIG_ID = 42
// This hands over this PAGE_TSCONFIG_ID to all flex fields that have this foreign_table_where marker.
// This is a contradiction to the "usual" page TSConfig flex configuration that should be done for single flex fields:
// TCEFORM.theTable.theFlexfield.theDataStructure.theSheet.theField.PAGE_TSCONFIG_ID = 42
// The below code is a hack to still simulate the old behavior that is now deprecated.
// @deprecated since TYPO3 v8, will be removed in TYPO3 v9
// When deleting this code and comment block, the according code within AbstractItemProvider can be removed, too.

But first of all this is for flex forms and second there is not description of a "new way".

EDIT 2:
"TCAdefaults.[table].pid = XY" does not work in this case, either.

Hey Robert, your title says "###PAGE_TSCONFIG_UID###" – the U is not correct here. Maybe it's just a typo why it's not working?

Anyway.. I know not very much about the topic but maybe you find some useful stuff on github with this search? https://github.com/search?l=PHP&q=PAGE_TSCONFIG_ID&type=Code&utf8=%E2%9C%93

Hi, Josef, thanks for your comments, the search list is really useful!
The "u" is just a typo since I typed from memory, not copy-paste.

Most of the scripts that can be found use the markers in connection with the foreign_table_where param, but I need a solution for TYPO3 8 and "addRecord". The old wizard used to work perfectly fine, but the new one that came with the formEngine, removed some of that functionality and changed other parts (like overwriting the whole list, once the added record is saved - which is ... NOT GOOD!!!)

Found this:
https://git.higidi.com/TYPO3/TYPO3.CMS/commit/02490e995e5a694a9256ffc928c186859ee74a55

This is actually working! Yeah!
But the documentation needs a good example, the currently provided text does not explain cleary, how this feature can be used. Will follow.

For reference purposes here some code:

// TSconfig

[applicationContext = Development]

TCEFORM {
  tx_rwfm_domain_model_address {
    categories.PAGE_TSCONFIG_ID = 19
    locations.PAGE_TSCONFIG_ID = 4
    persons.PAGE_TSCONFIG_ID = 9
  }
  tx_rwfm_domain_model_addresscategory {
    addresses.PAGE_TSCONFIG_ID = 3
  }
}

[global]

// TCA

return [
  'columns' => [
    'addresses' => [
      'config' => [
        'addRecord' => [
          'pid' => '###PAGE_TSCONFIG_ID###'
        ]
      ]
    ]
  ]
]

• the "pid" is the id of the child page that is referenced through the mm-field, here: categories or addresses respectively. The 'pid' of the parent table is completely irrelevant (this is, where the docu is most confusing IMO).
• on trying several condition scenarios, it turns out that "hostname" will not work, "applicationContext" does.