Insert attribute with TagBuilder class C#

I'm wondering if there is a way to insert an attribute inside a script tag without using the MergeAttributes or MergeAttribute method of the TagBuilder class.

I have a list of attribute data:

AttributeDataList = new List<AttributeData>
{
    new AttributeData()
    {
        Key = "",
        Value = ""
    }
},

And in my helper, in charge of creating the script, I have this check:

if (script.AttributeDataList.Count > 0)
{
    foreach (var attr in script.AttributeDataList)
    {
        scriptTag.MergeAttribute(attr.Key, attr.Value);
    }
}

Now suppose I want insert the "async" or "defer" attribute without using "async=async", how can I do that? The TagBuilder class seems not offers such method, and what I obtain is async=""

I was thinking to use a regex, and replace the '=""' with " ", but this pattern @"=\\""" seems not working.

There is a solution for that? Many Thanks