Your question suggests that you know the tag name for the citation you want to insert.
If that is the case, then you can insert a citation field manually as follows. Let's say the tag name is Doc14
Use ctrl-F9 (or cmd-F9 or fn-cmd-F9 on Mac, depending on your keyboard setup) to insert a pair of the special field code braces, so you see
{ }
Between the braces, type "CITATION " followed by the tag name, so you end up with
{ CITATION Doc14 }
Select the field, and press F9 to update it.
NB, this is is not quite the same way that Word uses to insert a CITATION field. Word inserts something called an "sdt", which contains a { CITATION } field but which arguably makes it harder to modify the field.