The ICP Laser page should have a Laser ST Time, or STart Time that you can set, and this will automatically start the laser at the set time before impact. 
 Also missing the TRNG/CMBT option at this stage  - but TGT Pod is still in dev.   
I see people starting to lase as soon as they drop the bomb a lot on YT sim vids, but you are supposed to allow the bomb to gain ballistic energy through free-fall before laser activation to ensure it has gathered as much momentum as possible to enable adequate manoeuvering to impact the lased point.  
 Hence why you drop in CCRP - as this gets it pretty close to start with so you don't need to start to lase 1 sec after release.     
Edit: I also believe cofcorpse is correct above, as you fire the laser by 'depressing' the trigger 1st stage - i.e. generally in aviation terms this means to hold it down, not just a momentary press and release. But I am sure some current techies can confirm.