Hello Ricardo,
using metadata, you cannot put default value for any field.
For seach requirement, implement WD_BADI_DOMODIFYVIEW BAdI (Enhancement Spot WD_BADI element definition) for the Web Dynpro Component (/SAPSRM/WDC_DOTC_ASL_SU_D) and its concerned view (V_ASL_DOTC_SOS) used as BAdI filter.
You code should be like this:
DATA: lr_checkbox TYPE REF TO cl_wd_checkbox.
lr_checkbox ?= view->get_element( 'SOURCES_OF_SUPPLY_ITM_RELEASED_EDITOR' ).
IF lr_checkbox ISBOUND.
CALLMETHOD lr_checkbox->SET_CHECKED
EXPORTING
value = abap_false.
ENDIF.
Regards.
Laurent.