The simplest way to name a source is to do just that. The information on the source document is not copyrighted just the image, method of presenting the information. A source is valid without an online link. What should be done with a source is to provide enough information so that someone else can find the information again to verify what you have stated about the person. It's the document and the information in it that are important, not the location of the information
There are several places where 1921 census is available at no charge, we wouldn't put National Library of Wales in Aberystwyth as the source of the information because we went there to view records at no charge, the source is the document.
I have a book that has detailed information about an ancestors settlement in Upper Canada in 1828, when I cite the source, I give the book title, date of publication and the publishers name etc. so other people can find the book in a public library, I don't say it's on the top shelf of the big bookcase in my living room.
I know that my great grandparents lived in the same house in 1921, that they lived in 1911, this is what the census record would say.
1921 Census of England and Wales, 2 Chapel St, Blakenall, Walsall, Staffordshire, England.
Then you can transcribe the information; adding page numbers or other similar information
Leonard Ross age 48, grocer and coal merchant
Sarah Ann Ross, age 46, at home
Alfred Thomas Ross, age 21, apprentice
etc, etc for the other children