How to append only specified columns?

Actually you can't select columns during the append step, but you can select prior to, or after, the append.

What I meant, is that you can either:

• remove unwanted columns (select the columns, right-click and choose "Remove", resulting in code using Table.RemoveColumns), or
• select required columns (select the columns, righ-click and choose "Remove Other Columns", resulting in code using Table,SelectColumns).

In below example, from Source table with columns1-10, columns1-3 result after selection/removal:

let
    Source = #table(10,{{1..10}}),
    #"Removed Other Columns" = Table.SelectColumns(Source,{"Column1", "Column2", "Column3"}),
    #"Removed Columns" = Table.RemoveColumns(Source,{"Column4", "Column5", "Column6", "Column7", "Column8", "Column9", "Column10"})
in
    #"Removed Columns"

So I was reacting to "I have manually deleted the unnecessary columns after the append" with the intention to state that you don't need to indicate which columns must be deleted; instead you can also indicate which columns need to be kept.

It was not my intention to state that you would be able to select columns during the append step; you can't.