When adding a species to the upload window you must always choose an option from the drop-down list. The fewer letters you introduce, the longer the list. The video below shows how to choose Entacmaea medusivora. As you can see, initially the ID box is greyed out. When you choose a taxon it turns green and the ID number for the taxon is shown (the same ID as in the WoRMS database).
There are around 500.000 taxons in the website’s database, of which 230.000 are accepted or valid taxons. If you have a slow connection the validation process may take a little time, so please wait for the list to fill.
See also this video, showing the selection of Amphipoda.
Sometimes the use of language plugins or automatic translators can cause issues. Try deleting cookies and begin again.
Remember to wait for the complete list to appear before choosing a taxon.