This is an excellent feature - when you click on the link to search MyHeritage on a FamilySearch Family Tree profile, there is a link to "Attach source to FamilySearch" for a record on MyHeritage.
Here is the process I used today to test it out:
1) I went to the Amos Underhill (1772-1865) profile on the FamilySearch Family Tree (he is my 4th great-grandfather) - this was a random selection. Here is the top of his profile (two screens):
2) I clicked on the MyHeritage link in the "Search Records" box on the right side of the screen. On MyHeritage, the "Summary" list of search results appeared. [Note that the default is a "not exact" search which results in over 600,000 matches.]
Since Amos died in 1865 in New York, I knew he wasn't in the census after 1860, nor was he in an England & Wales census. I knew he was in the 1860 U.S. Census, so I clicked that link on the screen above:
3) I think he is the first match on the list, so I clicked on that and saw the record summary and the record image:
4) I went back to the Amos Underhill profile on FamilySearch Family Tree, refreshed it, and scrolled down to the "Sources" section. There was the source citation information from the MyHeritage record for Amos Underhill in the 1860 U.S. Census:
FamilySearch essentially copied the information from the MyHeritage source information and crafted a source citation from it:
The source citation crafted by FamilySearch is:
This source citation is:
1860 United States Federal Census, Population Schedule, Erie County, New York, Aurora Township: Page 463, Dwelling #636, Family #626, Delos Underhill household; digital image, Ancestry.com (http://www.ancestry.com), citing National Archives Microfilm Publication M653, Roll 752.
This record is also on FamilySearch in the 1860 U.S. census collection. The source citation for this record on FamilySearch is:
"United States Census, 1860", database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MCW2-MM8 : 30 December 2015), Amos Underhill in entry for Delos Underhill, 1860.
6) This is a tremendous feature that FamilySearch and MyHeritage have created - it will help researchers add source citations to FamilySearch Family Tree profiles. However, the MyHeritage source citations are not Evidence Explained, or even FamilySearch quality, and leave a lot to be desired.
7) My guess is that FamilySearch will work with Ancestry.com, Findmypast.com, AmericanAncestors.org and perhaps other record providers to create a similar link to place on search results for records accessed from FamilySearch. I hope they do. I also hope that FamilySearch tries to standardize the source citations obtained from the search providers, and improves their own source citation.