Typically in reinforced concrete beam design the lower half is your tension zone and the upper half is your compression zone. The induced tensions are taken by the bottom reinforcement in such a beam since steel bar works effectively in tension. Concrete is weak in tension but strong in compression and normally is working in compression in the top half of the beam.
In reference to your photograph, you most likely refer to a capping beam which should be integral with the slab above. Over the window there could be a 'drop' reinforced beam or a lintol over the window, one course of blockwork and then the capping beam.
To answer your question. A structural engineer would probably accept a small hole within a beam within the neutral zone of the beam which is that between the main tension area (the bottom reinforcement) and the main compression area (the top of the beam) which in your instance would be include the slab. Try and find where the bottom of the concrete bean is located. The bottom reinforcement would be about 2" plus bar diameter above that and any hole that an engineer may permit would be at least 2" above the bottom bar reinforcement (near middle of the beam).
That is very basic guidance. You must not damage the bottom steel reinforcement for sure. A small hole is less than 1"diameter. Every structural design has differences so if in doubt take professional advice.