In general, you would use Tools->Insert Instructions from File
You minimally need a PLC designation in the top of the text file
PLC BX-DM1E
(or whatever)
followed by your code.
The dialog lets you decide where you want it to go. If you put your block cursor at the rung you want to insert it BEFORE, then do the Tools->Insert, the dialog should default to that position (you can tweak the location all you want). Even if it shows up somewhere else, do a CUT/PASTE once it's in Designer.
but you could enter it from Ladder
>= contact (STRGE) followed by a < contact in series (ANDLT) going to an OUT coil
-] >= [------] < [---------(OUT)