쓰세요 is order so this means 'please, write.(something)' 써서 주세요 is combined two verbs.(쓰다 and 주다) this means 'Write and give it ( to someone ).'

쓰세요 ----> 예) 답을 쓰세요. It means that just write something on somewhere such like on the paper. 써서 주세요 ----> 예) 이 문서를 작성해서 저에게 주세요. It means that please give it back to me after filling .You can be asked about fillng when you visit the doctor office at first time from the receptionist

여러분 정말 감사합니다.