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I don't quite understand your need but okey.

First if you need precise margin then first make image without it, do your stuff and then add white margins by canvas size - Image/Canvas size. Choose Relative and first add 1cm white (there is option at bottom) to one side then 2cm to other (experiment with those block to get right adding diretion or tutor on canvas size).

But before that make new image with your pattern alone then go Edit/Define Pattern and define it.

Go to your project. Then take Paint Bucket Tool from tools (under Gradient tool). In option replace Foreground to Pattern and in pattern preview choose your maded patern. Click on layer which you want to fill. Done.

And now you can add your white margins as i described before.