When dealing with multi-select pick list, simple operators like "+" and "," will not work if you want to update this form of field in an object.
In order to update this field to take into account multiple options, the operator to use will be ";", and it will look something like ' <data pill A> ; <data pill B>', if you will want to have this field updated to reflect the multiple options. This will only work if <data pill A> and <data pill B> are already pre configured as options when creating a custom field that is of the type "Multi-Select Pick Lists". Workato will not be able to create these options via our recipe. If it has been pre-configured, Workato will be able to facilitate multi selection through ";"