I don't see any way to add Data Labels to the cumulative percentage line.
The Pareto Line (as Excel calls it) does not seem to be a data series at all, but something internally calculated (like Trendlines). You'll just have to create the chart as a combo chart, which means you will need to create a column of data (i.e., a data series) that represents the cumulative percentage line. This I found on the information superhighway: https://www.wallstreetmojo.com/pareto-chart-in-excel/
Microsoft introduced a handful of new chart types in Excel 2016 and later, including Pareto charts, Waterfall Charts, and a few others. It's nice to have these chart types native in Excel, but unfortunately these new charts are incomplete. You have less formatting features and options compared to regular charts, and VBA can't do much with them.
I wrote about Pareto Charts on my blog, and I've written software to create Pareto charts.