As with mattmountz, I cook the first 3-4 hours as low as the smoker will cook — on mine, it’s a “smoke” setting, which is supposed to be 180-200, but anything around 200 will work. This is because pellets smoke more at lower temp’s and less at higher temps. 2-4 hours at a low temp should ensure enough smoke that you avoid the “no smoke flavour” complaints that often characterize pellet smokers.
Then bump it up to whatever temp at which you normally prefer to smoke briskets. I’m a low and slow guy, so I use 225-250. Others go faster at 275-300 or even higher.