There is a tab of ‘export citation’ to download the RIS file for EndNote referencing. You will see  page like this.
You can choose to just export the citation or with its abstract. After that a RIS file (.ris) will be downloaded, and you only need to click the file, choose your existing library, and finish! New reference is now in your library. Easy peasy!
Other than RIS format, EndNote is also able to import .enw files. Just perform the same action as .ris files.
The layout of each internet sites may be different, so here I give some other examples.
J-Stage (click the RIS text)
ASCE (go to ‘Tools’ then choose ‘Download Citation’)
SpringerLink (click ‘Export Citation’ then see below)