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Unless you create a new parameter within the reference line window, any measure you want to use for your reference line needs to somehow be in your viz. This includes your row or columns shelf, or the majority of your marks cards (not the tooltip). If you don't want to actually display it, you may be able to put it on the Detail mark. However it might change the level of detail of your viz so be mindful of that.
Once you have it somewhere on your viz, you can select it on the reference line window.
Welcome! Happy to help