To make a required enum field in Swagger UI for a .NET Core API succeed without user input:

1. Use OpenAPI Specification (OAS) in your API documentation.
2. Set a default value for the required enum field in the OAS.
3. This default value allows the Swagger UI to recognize the field as filled, satisfying the requirement without user interaction.

The default behavior of Swagger UI is to require the user to make a change to the value of a required enum field in a form before the form can be submitted. This is because the enum field is not populated by default, and Swagger UI needs to know what value to submit to the API.

There are a few ways to work around this behavior. One way is to set the default value of the enum field to the first value in the enum. This will allow the user to submit the form without making a change to the value.

Another way to work around this behavior is to use the @ApiModelProperty annotation to specify the default value of the enum field. This annotation tells Swagger UI to use the specified value as the default value, even if the enum field is not populated.

Here is an example of how to use the @ApiModelProperty annotation to specify the default value of an enum field:

public enum Size
{
    Small,
    Medium,
    Large
}
 [ApiModelProperty(
    "The size of the item",
    default=Size.Small
)]
public Size Size { get; set; }

With this annotation, the default value of the Size enum field will be Small. This means that the user will be able to submit the form without making a change to the value of the field.