Need RO set up help...

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That's all you have to do.

Get a Tee with compression fitting that will work on the grey line. Could use the existing valve as your only shutoff, but handy to get a second valve for after the tee for servicing your ro without interferring with the sink's tap, though not really required. For when you move just replace the grey line (now with the tee) with another continuous piece.

If you don't mind going without water for a bit just turn off the valve, take out the grey line (between the valve and the faucet), grab a piece of the water line for your ro unit, go to HD depot the show them what your planning. Since the grey line is probably bigger than the one for the ro, you'll need a tee and reducer.