I finally got the solution:
In the button event implement the following, to access the node and finally the id-value:
method ONACTIONZSS10_15_ONDELETE .
DATA ls_cust type wd_this->element_IT_Cust.
DATA lo_nd_cust TYPE REF TO if_wd_context_node.
DATA lo_el_cust TYPE REF TO if_wd_context_element.
" Get the selected element
lo_nd_cust = wd_context->get_child_node( name = 'IT_CUST' ).
lo_el_cust = lo_nd_cust->get_element( ).
" Get the attributes of the node-element
lo_el_cust->get_static_attributes(
IMPORTING
static_attributes = ls_cust ).
" Call the delete-function
CALL FUNCTION 'ZSS10_15_CUST_FM_DELETE'
EXPORTING
custid = ls_cust-ID
.
endmethod.