Normally with cruiser style drums, there's two ways it can be done.
The cheapo way is that there's an arm that sticks off the drum brake body with a hole in it, and you use a screw-on strap to fasten it to the left fork leg (rather like how most coaster-brake arms attach).
The expensive way is that most drum brakes have some sort of slot cut into the anchoring arm. You can weld a tab on the left fork blade that sticks inside (towards the wheel) and when you bolt the front wheel on, the slot in the drum brake fits over that tab to keep the drum brake from rotating.
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