How to match "Go struct tag"

I want to develop a code completion plugin. It can provice code completion for “golang struct tag”. 
Now I don't know how to write it.

public class MyCompletionContributorTag extends CompletionContributor {
    public MyCompletionContributorTag() {
        extend(
            CompletionType.BASIC,
            // it doesn't work.
            PlatformPatterns.psiElement(GoTypes.STRING).withParent(GoTag.class),
            new MyCompletionProvider()
        );
    }
}

Please help me. Thank you.