You can apply for DSA when you apply for student finance online.
If you are not applying for student finance but wish to apply for DSA you can fill in a full DSA 1 form.
You can access the forms here https://www.gov.uk/government/publications/disabled-students-allowance-application-forms-and-notes-for-2022-to-2023-full-time-students
The Accessibility Team are here to advise and support you throughout this process. You can find more information at How do I apply? | StudyHub (fxplus.ac.uk)