What is the proj4string of the Lambert azimuthal equal-area projection?

9820 is the EPSG code for the laea projection definition. You can find it at https://www.epsg-registry.org/ under retrieve by code. This code numer has nothing to do with the commonly used EPSG codes like 4326 and 3857. These include projection parameters specified for a certain country or region.

For laea centered on Europe, the code is EPSG:3035, with these parameters:

+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

As you can see, the center is at 10°E and 52°N, which is not exactly what your source recommends. If you want those, you can build a custom CRS with parameters:

+proj=laea +lat_0=53 +lon_0=9 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

The x_0 and y_0 have no effect on the projection, they just shift the coordinates to avoid negative coordinates.

The EPSG code for LAEA is 3035. Further details available here