manipulation – Subtracting the background from the Lorentzian


I am plotting a dataset which is a Lorentzian, for the fitting, I need to subtract the background. I want to subtract a line from the dataset which connects the first and last points of my dataset.
Here is the dataset Data. I do not know how to proceed with it.

Data = Import["Documents\24.asc"];
Ampvsf1 = Data[[All, {1, 4}]];
graph = ListPlot[Ampvsf1, PlotRange -> Automatic]

I am attaching an image to make myself more clear

enter image description here