If you can share the purpose of creating external user, then it would have been easier so suggest a more concrete solution. Anyway, assuming that you need it for DWA ( Direct Web Access ) , here will be my suggested steps for this.
1. Create a new Operator ID that starts with the reserved word External.
2. On the Operator tab, specify an access group, organization, and division that provides the external operator with the ability to complete the external assignment. For security reasons, reference an access group that provides only the minimal RuleSet Versions that the external user needs to complete the assignment.
3. In the opertor id rule form , under Security tab. Set the License Type field to Invocation. Leave the Use external authentication? check box blank.