Do you know if its possible to add a custom text like this one (paid in full), on records? I want to add a text that looks like this on a project task

I don't think so, but instead you can create a banner or a text using inline html for status

I think the closest you can get is adding a banner in a UE beforeload something ilke this:

const beforeLoad = (context) => {
            if (context.type === context.UserEventType.VIEW) {
                let messageTitle = "My Title";
                let messageBlock = "My important message"
                context.form.addPageInitMessage({
                    type: message.Type.INFORMATION
                    , title: messageTitle
                    , message: messageBlock
                });
            }
            return true;
        }

use inline html with a standard control and add some css to style it