This question already has an answer here:
I would like the backend left menu sidebar be collapsed by default. Yes, I can click the "Collapse menu" button every time but is there a way to have WP open up every time with it collapsed by default?
I tried this solution but it didn't work for me: How to Make admin Sidebar Menu always be Collapse by code
I'm also trying to apply this to a multisite website, not sure if that's why the code from the link didn't work.
Any ideas on how to achieve this?