Interactive box selection

Click the Open Data Source Manager and Add raster layer and select the file /geodata/raster/s2/Subset_S2A_MSIL2A_20170619T_MUL.tif.
Raster --> Extraction ---> Clip Raster by Extent
Input file (raster): Choose a loaded file from the drop down list or click Select to browse for a file.
Output file: click Select to choose a new file name for the subset image.
Define the clipping extent by clicking... and Select extent from canvas The clipping mode is now active. Hold a left click on the map canvas dragging a box of your region of interest.
Adjust the extent coordinates in the text file rounding the coordinates (upper left and lower right).

Vector polygon masking

Load the vector file \geodata\vector\gadm36_DEU_4.shp. These are the level 4 administrative boundaries of the country Germany downloaded from https://gadm.org/index.html .
Right click on Layer name in the Layers Panel Properties --> General. Change the Data source encoding from System to UTF-8. Click UTF-8.
Right click on Layer name in the Layers Panel and open the Attribute table.
Click and select by the expression: "NAME_4" ='Göttingen'. Select.

Right click on Layer name in the Layers Panel . Export --> Save Features as... Save as.... Fill the form as shown below.
Click the checkbox Save only the selected features.
The target coordinate reference system (CRS) is WGS84 / UTM zone 32 (EPSG:32632).

Make sure that the CRS is the same like the image file (here WGS84/UTM Zone 32, EPSG:32632)!

Input file (raster): Choose a loaded file name from the drop down list or click Select to browse to a file
Mask layer: Select the exported vector file (s. above).
Clipped (mask)): click Select to choose a new file name for the subset image.
Switch the checkbox on: Crop the extent of the target dataset to the extent of the cutline. All image data outside the polygon will be masked.
Switch Keep resolution of output raster on.
Choose output data type {{typed|text=Uint16}.
Click Run in Background.