Yes, you can! Twitter Cards will work with all versions of ONTRApages!
Copy the code on that page where it's nicely formatted, or use this code block:
<meta name="twitter:card" content="summary" /> <meta name="twitter:site" content="@flickr" /> <meta name="twitter:title" content="Small Island Developing States Photo Submission" /> <meta name="twitter:description" content="View the album on Flickr." /> <meta name="twitter:image" content="https://farm6.staticflickr.com/5510/14338202952_93595258ff_z.jpg" />
Paste that code into your ONTRApages page on the Settings tab in the "Custom header code" section, as shown here:
Edit the code as shown, with your own Summary, your Twitter address for the "twitter:site" value, a "headline" for the "twitter:title" value, and your description in the "twitter:description" value. To add an image, paste the image URL into the "twitter:image" value.
Save and publish the ONTRApage.
Now, go to the Twitter Card Validator site and enter the URL to your ONTRApages page. The Validator will show you a preview of the Twitter Card:
Include the ONTRApages URL in a Tweet, find your Tweet in your feed, and click on the date / time to expand it. The card will show below the Tweet!
Other Twitter Card Types work too; here's an image of the "Summary Card with Large Image" Card Type:
Twitter Cards show up only when you expand the Tweet by clicking on the date or time. They do not show up in the normal feed. This is a limitation of Twitter and not a limitation of ONTRApages.