How can I use built-in effects processor that goes to main to send to monitors?
Some Behringer mixers have dedicated mon fx sends like the below (from the PMP1600):
Other Behringer mixers (e.g. the XENYX X1204USB) don't have that option and here you have to patch back the FX send into an unused channel and from this channel send to the monitors in question (but DON'T send to the FX send - it will create a feedback loop!).
Note that Aux Return 2 often used as a way to disable the internal FX processor, so make sure you don't put anything into that (source, p. 5)