How do i add drop down field for credentials in the plugin im devloping

I am some what new to extending Jenkins and was wondering how would i generate the code for adding a credentials drop down like the one in this picture.
_{(source: jenkins.io)}

I have looked at some examples and noted they used this jelly code

<f:entry title="${%Credentials}" field="credentialsId">
  <c:select/>
</f:entry>

But when i implement this my Credentials line in the build job settings dose not contain the drop down.

Note figured it out. Posting answer for others benefit. In order to dynamically fill the list you must create a doFillCredentialsIdItems like the method bellow along with adding a import to the credentials plugin.

public ListBoxModel doFillCredentialsIdItems(@AncestorInPath Item item, @QueryParameter String credentialsId) {
        StandardListBoxModel result = new StandardListBoxModel();
                        return result
                .includeEmptyValue()
                .includeAs(ACL.SYSTEM, Jenkins.get(),
                 UsernamePasswordCredentialsImpl.class)
                .includeCurrentValue(credentialsId);
        }