Normally when using the GBU-12 you're supposed to use IR grenades thrown by friendlies or other means of "laser" marking, wether that be your own laser pod or a laser designator held by a spotter.

However, even not using these methods you can simply "lock on" with the GBU if you first select a target with cannon then switch to GBU and drop the bomb roughly in the direction of the target.
Video for evidence: https://streamable.com/uqg02
As you can see, as long as I can select the target with cannon by pressing "T" or "R" it keeps said target when you switch to the GBU, no further user input is required, all you need to do is drop the bomb in roughly the same direction/area as the target and it'll automatically lock on.

This is seriously impacting PvP gameplay (in a negative way), especially in "King of the hill".