Hi @mapo85,
Happy to help you!
In fact, we have two ways of adding a custom field, like the one you are looking for, for your donation forms.
You can add that by using one of our Pro add-ons called Form Field Manager, that allows you to create custom fields for your forms. As it’s a paid add-on, we cannot support it here at www.remarpro.com, but we’d support you via priority support.
The second option (which is free), is to use one of our code snippets available on our snippet library.
This code here https://github.com/impress-org/givewp-snippet-library/blob/master/form-customizations/custom-fields-plugin.php allows you to add a custom field on your donation forms, but you’ll need to customize it to your needs.
Please let us know if you have further questions on this or need any additional assistance!