Buttons can be very useful, but 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 bit smaller, so it doesn't stretch over the whole width. Note: you can give it any size, not only in %, 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>
For further help, contact firstname.lastname@example.org