The combined help of both of you solved that problem step. Thanks a lot @mallika and @Paul_Danyliuk!
This is what I got to work well:
ModifyRows(thisRow, articleReferenceTable.blueColumn, ListCombine( thisRow.blueColumn, newValue ).Filter(CurrentValue!="").Unique() )
Now the only remaining problem is, that the newValue should be replaced by a popup for creating a new record, with Activate(AddRow()). The result of that new record should be placed as a referenced element among the multi-values in thisRow.blueColumn.
But here the modal window refuses to open to let me create a new record.
This is what I tried for the button:
ModifyRows(thisRow, articleReferenceTable.blueColumn, ListCombine( thisRow.blueColumn, Activate(AddRow([Detail view of articleReferenceTable])) ).Filter(CurrentValue!="").Unique() )
This is an articleReferenceTable. The blueColumn in the table references to elements/rows of the same table. The idea is that an article may reference to one or several other articles, which are also rows in the same table. and the blueColumn is used to store all other articles that the article in thisRow references.
The logic is, that in the current row record of the detail view, one searches among existing articles to link the article to. If the article we wish to link to doesn’t previously exist, then the button should open a modal to enter the new record, and when it is saved, it is placed among the other elements in the blueColumn cell of thisRow in the current record, from which the button was pushed.
I managed to get the Activate(AddRow()) to work correctly per se, and get it to replace the previous value(s) of the blueColumn. But now as I want it not to replace, but to append the returned, newly created row, then an “Action error” element appears.