Create your text campaign, or update your existing campaign.
More information on How to Send a text using Campaign Builder.
Take note: Url will only work if you are currently on Free and Optimal Access plan.
In the message content add the Merge Field ~url~.
Then, add a Value Pair. In the left field, put in the word 'url'. In the right field paste your long link.
Your long link will automatically be shortened, and will look like this.