[dev] How can OTRS ITSM3.3.7 get the customer Grading for the tickets?

Dear Professors: I wanna ask for idea that: what can I do to the OTRS ITSM 3.3.7 to set the customer Grading for the tickets when we close them? We want to get the grade as the reference of the IT Service KPI index. So, does anyone have the idea, please? TKS a lot! BRS Walson Way