Store selected (highlighted) row from a visual as a value

Well I noticed that. But for a couple of reasons, I wanted to have it as its own value. First because I thought it might help with processing speed of the Power Automate flow since the table in question has 1M+ rows. But more importantly because I haven't been able to figure out how to prevent the flow from working when a user hasn't selected a value yet. I don't want the flow to do its thing for the million rows - just for a single selected row. I've played around with trigger conditions on the flow button without success. And I can't use the transparent card trick to hide the button based on a condition because the card will be on top and prevent clicking the button when the condition is met. So my thought was to write the selected value to its own table so that I would then have a single row table to use for the flow.

I feel like it must be possible because if I were to place that measure on the canvas in its own visual like a table, it works correctly. But when I place that measure inside the original table it gets evaluated at each row of the visual and then doesn't work correctly. So I was thinking it was just something I wasn't understanding about context that I could work around using a CALCULATE function with the appropriate FILTER condition on the measure, so that I could then use that to create a new single row table. Any ideas for that?