Find-what zone: (?-s)^(.*)/
Replace-with zone: \1?id=
Search mode: Regular expression
How it works:
On a single line, match any characters until a final / is encountered; remember everything that occurred before that final / as group #1–because of the parenthesis wrapping it. Replace the matched text with the contents of group #1 (the effect of this is to repeat everything except that final /), then insert ?id= into the text. Because what originally came after the final / is not involved in the match, it remains as-is after the insertion of ?id=, giving the final result.