All donation forms have an automatically generated short link provided for you.
The first way to find the link is to click on one of the form types in the navigation menu at the top, such as Fundraising > Text to Donate. If you have already created forms of this type, you will see the form information and it's short code, called the Public Link.
You can also find the sharable link by clicking on the keyword. Once in the form page, find the Display Links box below the menu on the left-hand side. In this box you will see a link titled Shareable Link to This Form.
NOTE: This "igfn.us/xxxxx" link is the same link that is sent out donors when they text in to your keyword.