Most troughs already have a drain hole to enable the troughs to be emptied and cleaned , when they were originally used on farms as water troughs.  However some don’t, but troughs can easily be drilled.  (It is best to drill them at either end, towards the bottom of the side walls,  so that the holes don’t get filled up with soil).  It is advisable to place a good amount of “crock” in the bottom of troughs to act as drainage for plants.  (A good 3”-4” of broken up terracotta or rubble is ideal.  Not limestone, as some plants don’t like lime).  We use multipurpose compost for planting mixed with good sandy soil.   If you are using just compost, it is worth mixing 1 part sharp sand with 2 parts compost.  We also dress the mix with a few handfuls of general fertilizer. )