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For the first task of just showing regional borders, one way to show borders in Tableau is to adjust which base map layers are visible. You can access the layers list from Map -> Map Layers...
The second task of identifying the area within a set distance of the border is a different matter though... for that you will need to have polygon data to use in the calculation. If you are working with standard US borders (state, county, census tract, etc.) you can get those shapefiles from the US Census boundary files web site. If you will always use the same distance buffer from the borders, you can create a buffered dataset quickly using QGIS (a free and open-source GIS package - there are a ton of tutorials if you just search for 'QGIS create buffer').
If you just need the area (e.g., how many square KM are covered) you can calculate this area easily in QGIS and assign it as an attribute to each of your buffered regions.
If you need to select features that are within the border, that buffered shapefile could be added to Tableau and used with the spatial intersection feature introduced in version 2018.2. Or, if you need the buffer to be dynamic (e.g., adjust from 20km to 10km or any other distance), you would probably want to explore options for using the Tableau SQL Server spatial support introduced in 2018.1 and 1) generate the buffer on-the-fly using a parameter and some custom SQL, and 2) either use that new geometry with a spatial intersection join OR use custom SQL to perform the spatial intersection. The one you choose would probably depend on your data types (e.g., point, line, or polygon) and how much you enjoy writing SQL.