Unlike adding Free School Meal (FSM) Eligibility, you can't record a student specifically as being Universal Free School Meals (UFSM).
This is because UFSM is automatically applied in the background to all students that are enrolled in a Reception, Year 1 or Year 2 year group curriculum grade. This will automatically feed through to the census and Meals and payments areas so they'll be charged the UFSM prices.
If the student is not flagging as UFSM that should be, check they are in the right year group from the Enrolment section of their student profile. If they're not, you can fix this following these instructions.
Next, check the curriculum grade the student's year group is linked to. Go to Students > Enrolment > Year Groups and click on the year group to make sure the curriculum grade is correct.