FTP File name Pattern - OIC GEN 3

I am trying to move a file from source FTP location to Target. so when the source file name is "test.txt" i am trying to rename the target with "test_%yyMMddHHmmss%.txt". 

Which function should I use in the expression builder for the file name?

I see an accepted answer "Did you type the expression or built it from UI? Once the format-dateTime is dragged and dropped, add a sibling after to the argument shown. Now you will have 2 arguments - 1st one current-dateTime and second one the string expression above." But I don't understand how to use it.