My question is, how can i add a fixed position to the background of a page or a content inside the page?
If I understand your question correctly, It sounds like you may want to explore the Pin feature.
Look for this graphic at the top of the Muse work area:
It only appears when you have an object selected (Image, rectangle, text, etc), and only has an effect when you resize the browser or, if your content is long enough, when you scroll the page.
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