Can you create a Go struct containing only the attributes received by the instance?

Question

I'm using the AWS SDK for Go to get parameters from SSM. In order to instantiate an SSM client, I have to pass a session struct with params such as region, credentials and endpoint.

&aws.Config{
    Region:      aws.String("us-east-1"),
    Credentials: credentials.NewStaticCredentials("foo", "var", ""),
    Endpoint:    aws.String(os.Getenv("SSM_ENDPOINT")),
}

These credentials are optional, and have their standard values be overridden when user input is provided. I've been trying to create a flexible solution for receiving those inputs, such as:

• Mapping inputs through a variadic function: Inputs would have to follow a specified order, can't really tell what each value's key is.
• Instantiating specific values of a larger struct: Other values get instantiated on their zero values.

A possible solution would be Marshaling and Unmarshaling a JSON with json:",omitempty", but it doesn't really seem like the best way to do it.