By using Givsum's Donation Widget you can add a "donate" button and allow your visitors to donate from your website. Copy and paste the code provided in your charity SETTINGS page, or use the code below by replacing [charity] with the charity's url slug.
<a href="https://www.givsum.com/charities/[charity]" class="givbtn">Donate now on Givsum</a>
The "script" tag only needs to be called once and can be placed anywhere relative to your widget button(s).
Use a "data-style" attribute with the values "full" or "none" to make the button full-width, or remove the default styling.
Use a "data-amount" attribute to pre-populate the donation amount field inside the popup. You can also set the donation to recurring by setting "data-recurring" to 1, 3 or 12 for monthly, quarterly or annually. Note that these pre-set values are not forced and the user can still change them.