Online sources and other types of sources are cited the same way as printed sources. In other words, the word allowing to find the source in the bibliography is included in the citation. The citation mentions the author, the publication date and the pages, e.g.
If the author data is missing, the reference is started with a corporate author (e.g. Finnish Institute for Health and Welfare, or Natural Resources Institute Finland).
If the year or date of publication is missing in the source, the author or publisher and the abbreviation n.d. (no date) are mentioned, e.g.
When referring to the same website, the sources are distinguished from each other with lower case letters attached to the date of publication, e.g.
URL addresses are not included in in-text references but mentioned in the bibliography.