<f:entry title="${%Test select}" field="testSelect">
    <f:select/>
</f:entry> 

// in Describable
private String testSelect;

public String getTestSelect() {
    return testSelect;
}

@DataBoundSetter
public void setTestSelect(String testSelect) {
    this.testSelect = testSelect;
}

// [...]

// in Descriptor
public ListBoxModel doFillTestSelectItems(@QueryParameter String testSelect){
    ListBoxModel items = new ListBoxModel();

    items.add(new ListBoxModel.Option("- none -", "", testSelect.isBlank()));
    items.add(new ListBoxModel.Option("Test 1", "test1", testSelect.equals("test1")));
    items.add(new ListBoxModel.Option("Test 2", "test2", testSelect.equals("test2")));
    items.add(new ListBoxModel.Option("Test 3", "test3", testSelect.equals("test3")));

    return items;
}