Hi Nitin,
i will give you one suggestion when you click on button this url you need to open right.
i have faced same problem last time. so at that moment i have done my code in do_prepare_output.
when ever your declare your button properties there is one property onclientclick.
CONCATENATE 'javascript:window.open( "http://YOURLINK.com:8000/sap/bc/gui/sap/its/webgui/!?~transaction=TCODE&~okcode=ICEXECUTE&sap-user=crm_remote&sap-password=sap_user&sap-client=300&IP_VBELV=' zvbeln '&~okcode=/00&");' INTO
lv_string.
ls_button-type = cl_thtmlb_util=>gc_icon_print.
ls_button-text = 'PRINT'.
ls_button-enabled = me->view_group_context->is_view_in_display_mode( me ).
ls_button-on_click = 'PRINT'. "#EC NOTEXT
ls_button-on_client_click = lv_string.
APPEND ls_button TO gt_button.
CLEAR ls_button.
try this..
Thanks & Regards,
Srinivask.