When trying to add a URL source through the research interface, it did not work (gives WikiRat error message), but going through the 'Browse' > 'Source' > 'Add Source' it did work.
https://wikirate.org/Research_Page?rp%5Bcompany%5D=Nittan+Co.%2C+Ltd&rp%5Bmetric%5D=Business+%26+Human+Rights+Resource+Centre%2BMSA+Statement+Homepage+Link&rp%5Bpinned%5D%5B%5D=company&rp%5Bproject%5D=UK_Modern_Slavery_Act_Research&rp%5Byear%5D=2018&view=slot_machine
found and fixed this bug. thanks.