Keywords are a tool to help indexers and search engines find relevant papers. If database search engines can find your journal manuscript, readers will be able to find it too. This will increase the number of people reading your manuscript, and likely lead to more citations.
However, to be effective, Keywords must be chosen carefully...
The journal usually asks me to put in the keywords as part of the upload of the manuscript. They often have a specific page for this function (to make sure they got your keywords correct) in the same way that they might want you to enter the names and order of the authors in a certain page.
In addition, I would make sure it is clear also in the manuscript I upload.