Synching iPhone to New Computer?

I just got a new MacBook. When I tried to synch my music from my iPhone, it said that since the iPhone can only be linked to one iTunes account at a time, that it will erase all music from my phone and replace it with the music on my computer (which is none because it is new). How can I get the music from my phone to my computer without having to erase everything from my phone currently?

Here you can find it

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