Is it possible to open a specific WMS layer in QGIS directly by entering the whole URL of this WMS? That is, without having to open a WMS connection first and selecting the layer in the tree.
Yes, you may do it with a few lines of python, see:
I would recommend testing in the python console in Q, if that works then you could create a simple Q plugin and include that logic for whatever layer(s) you may want to add in.
You have to give the endpoint URL to QGIS, then QGIS appends appropriate parameters to make the map appear.