Did you mean ICSE or IGCSE? Whatever be case, you have to select your board while filling the application form of JEE. In the academic section, there will be a option like 'School Board of Class 12th/Qualifying Examination'. Then select your board from the drop down list.

Here is a link for illustration : https://www.sarvgyan.com/articles/how-to-apply-for-jee-main

Thank you.