Add an add entry button to my Plugin in Craft CP
I believe what you need is the elementSelect field type which Craft bundles with the CMS. You can find the template of this field in craft/app/templates/_includes/elementSelect.html

Here's a quick example that will get you started:

{{ forms.elementSelectField({
    label: 'Select An Entry'|t,
    instructions: 'Select...'|t,
    id: 'entry',
    name: 'entry',
    elementType: craft.elements.getElementType("Entry"),
    source: '',
    criteria: '',
    sourceElementId: '',
    elements: '',
    storageKey: '',
    viewMode: 'list',
    limit: 1
 }) }}