If your kit extract is liquid malt extract (LME) I wouldn't boil it regardless if prehopped or not. It is sanitary in the can, and boiling it may contribute negative qualities such an increase in malt flavor and color and, if prehopped, a change in bitterness...basically changing the beer from kit design intentions.
Rather, boil your water for 10 minutes to sanitize, turn off heat, add LME (if you soak cans in separate hot water it helps them pour) and stir.
If using any dry malt extract (DME), sugar, or hops then add these to your water when its warm but not yet boiling, then do the 10 minute boil procedure as above. This will sanitize the water, DME and sugar, and get decent utilization out of the hops.
FYI - if adding hops and recipe has 60 minute addition, you will need to add approximately 2.5-3.0 X as much hops in your 10 minute boil to get similar bittering.