Here is one approach you could consider - we're checking if the "Reserved for" field is empty, and if it is - then we take the "Reservation Requested By" info and stamp it into the "Reserved for".
let table = base.getTable("Status Report");
let record = await input.recordAsync('Reservation Requested By', table);
if (!record?.getCellValueAsString("Reserved for")) {
let newValue = record?.getCellValueAsString("Reservation Requested By")
await table.updateRecordAsync(record ,{"Reserved for": newValue});
};