Have the user select the geometries they want to merge.
Use the Geometry Service or Geometry Engine (referenced by @KarenRobine1 ) to union the geometries.
Prompt the user to provide the values for the attributes.
Create a new graphic with the geometry from the union. At the same time, you can set the attribute values. You do not need to have a value for every attribute. Any attributes not included will be set to the default value for that field by the service when the graphic is added.
Make an applyEdits call to the feature layer and include the new graphic as an add. If you want to remove the features you used for the union, you can include them in the call as deletes.
Courtney Menikheim | Application Developer | @thecmenikheim