See external documentation: wxStyledTextEvent.

Use wxEvtHandler:connect/3 with EventType:

stc_change, stc_styleneeded, stc_charadded, stc_savepointreached, stc_savepointleft, stc_romodifyattempt, stc_key, stc_doubleclick, stc_updateui, stc_modified, stc_macrorecord, stc_marginclick, stc_needshown, stc_painted, stc_userlistselection, stc_uridropped, stc_dwellstart, stc_dwellend, stc_start_drag, stc_drag_over, stc_do_drop, stc_zoom, stc_hotspot_click, stc_hotspot_dclick, stc_calltip_click, stc_autocomp_selection

See also the message variant #wxStyledText{} event record type.

This class is derived (and can use functions) from:
wxCommandEvent
wxEvent