Be aware that you mostly want one call to action per page, so don't add additional buttons carelessly!
If you don't have donations with Campaignion, or want to add a button linking to another action or form, you can manually add a button within the editor of your textfield. You just need to click the "source" button and be allowed to use html in this field.
To make your link look like a button, it needs the same styles as all other buttons on your Campaignion installation.
All css styles for a button have a class: button.
So you'll get the same button styles as all other buttons if you add your link like this:
<a href="https://whereever-you-want-to-link-to" class="button">Your button text</a>
Two more things might be nice to have:
adding target="_blank" will open the site in a new window or tab (depending on the user's browser setting)
adding style="max-width: 25%" will make the button a tad smaller, so it doesn't stretch over the whole width. (You can give it any size, not only %, but also in px or em)
So in the end the code might look like this:
<a href="https://my-example-donation" target="_blank" style="max-width: 25%" class="button">Donate</a>