Add the feature layer URL to the web map as shown in the following example. The example works with a join. http://servername.domainname.com/arcgis/rest/services/sampleservice/MapServer/0
Before performing the join in ArcMap, click the Add Data button to add data instead of dragging the data from the Catalog pane to prevent adding the percent character to the table. Publish it as a map service.
Store the database table in the same geodatabase as the feature class before performing the join and publishing it to prevent adding the percent character to the table. The table can be created using SQL and does not have to be registered with the geodatabase if using a non-enterprise geodatabase table.