How to declare a ValueBuilder object to be a StringSet when doing an UpdateItem in Dynamodb using gosdk?

NOTE: Might seem like a duplicate of this but it's different because I need to do this via golang while that is in JS.

I want to do the following operation in goSDK on a dynamodb item:

 UpdateExpression: 'ADD socialAccounts :socialAccountId',
 ExpressionAttributeValues: {
   ':socialAccountId': {
      'SS': [socialAccountId]
    }
 },

Typically how that works out in GoSDK is:

expression.Add(expression.Name("socialAccounts"), expression.Value([]string{"socialAccountId"}))

However the SDK is not taking the array of strings as a SS(StringSet) type, but instead as a L(List) type.

Logged Error:

Error: ValidationException: Invalid UpdateExpression: Incorrect operand type for operator or function; operator: ADD, operand type: LIST, typeSet: ALLOWED_FOR_ADD_OPERAND

I'm unable to find any functionality in the docs that specify how to mention a type.

Can anyone help?
Why I want to do it this way:

1. I want this to work even when the attribute socialAccounts does not exist.
2. I don't want to use the SET opearation with list_append because that can introduce duplicate elements.

Use dynamodb.AttributeValue if you need to specify the type:

package main

import (
	"fmt"

	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/service/dynamodb"
	"github.com/aws/aws-sdk-go/service/dynamodb/expression"
)

func main() {
	val := (&dynamodb.AttributeValue{}).SetSS(aws.StringSlice([]string{"socialAccountId"}))
	expression.Add(expression.Name("socialAccounts"), expression.Value(val))

	fmt.Printf("%s\n", val)
}

Output:

{
  SS: ["socialAccountId"]
}

I prefer @Zeke Lu 's answer over my own.
But the following is a "push-comes-to-shove" method that should accomplish everything.

updateExpr := "ADD #SOCIAL :SOCIAL"
expName := map[string]*string{"#SOCIAL": aws.String(SOCIAL)}
expVal := map[string]*dynamodb.AttributeValue{":SOCIAL": {SS: aws.StringSlice(socialAccounts)}}

_, errUpdate = tablesConnection.UpdateItem(&dynamodb.UpdateItemInput{
	ExpressionAttributeNames:  expName,
	ExpressionAttributeValues: expVal,
	Key:                       key,
	TableName:                 aws.String(tableName),
	UpdateExpression:          aws.String(updateExpr),
})