This is achieved by moving the gearbox backwards/forwards as required. Be aware that the primary chain will have 'tight spots' due to slight eccentricity of the clutch drum and drive sprocket. I'd set the slack to be no more than 1/2" at its slackest spot, then check that there is still some play at the tight spots. The gearbox has a very large bolt across the top which passes through the adjuster eye bolt. It has a similar bolt underneath that the gearbox swivels on. Slacken both the large bolts then use the adjuster nuts on the eye bolt to slide the gearbox back until the chain is a bit too tight, then gradually move the adjuster nuts again to move the gearbox forward to the desired tension, then tighten all the bolts really well, and lock up the adjuster nuts. Moving the gearbox will also affect the tension on the rear chain of course so you need to finally check that and adjust as necessary bearing in mind that the chain will go tight as the suspension is compressed so it is worth doing this, for the first time with the rear shocks removed to identify the tightest position of the chain relative to the swinging arm and make sure you still have a bit of slack. Ride the bike a few miles and check again as things tend to move due to poor, original, manufacturing tolerances!
You don't need to read the next bit but, for the benefit of some, there is a technical reason why, when adjusting the gearbox and primary chain it is necessary to move the gearbox forwards to achieve the correct tension. This is because the torque at the rear wheel is greater (according to a conversation I had with Bob Newby of Bob Newby Racing - the transmission specialist). The rear wheel will therefore tend to pull the gearbox back so, to overcome this, it is necessary to ensure that there is no gap between the the cross-bolt and the back of the eye bolt tensioner, a gap which will be filled if the gearbox gets pulled back, then the primary chain will be too tight.
