If you are on FamilySearch.org, you find an item in their index that matches the person's profile you are working on in WikiTree. On the right side of the index entries is a column. In that column can be various types of links to further information -- a page for a description or transcription, a camera for a digital copy, a branch if someone has attached this info to their tree.
When you click on a page in that right-hand column, you will see material transcribed from the source about that entry. NEW - on the bottom right of that page are the words DOCUMENT INFORMATION with a drop-down arrow.
Click on the arrow and the information's source citation will appear. Included in that citation is a direct link in this form: (https://familysearch.org/ark:/61903/1:1:2WHF-KDN
Copy and paste the whole section of text, with that link, to the profile on which you are working. After being saved, the link will show in profile view and others can click to go directly to the information you have found! It's a great thing.
There is a similar text available on Ancestry.com describing sources. However, since Ancestry is a subscription site, not all WikiTree members can view material stored there.
I hope this answers at least part of your question! Go add some sources and try it out. -NGP