Creating Custom Checkbox and attaching a Error Message

Hi All,

        In the final step of shopping cart i need to place a custom checkbox and user tries to click ORDER button.He should get a Error Message asking to 'Select the checkbox'.

How to achieve this.

I createa a field in SPRO in header level structure and a check box in Webdynpro Component and do i need to write anything in BBP_DOC_CHECK_BADI

Component Name :/SAPSRM/WDC_SC_WZ_GAF_3ST

View Name : V_SC_WZ_GAF_3ST

Regards,

Krish.