html or php

I am not sure what are you trying to do, can you explain it more clearly?

The HTML code should work out-of-box, the PHP code you cannot use inside the textarea for security reasons.

I suppose you want the button to be shown on this page https://ibb.co/gXry2p? If so then it seems to be working fine for me, except the code you pasted for the payment button has only a form and a couple of hidden fields, so there is nothing to display for the user, try adding inside the form some submit button like

<input type="submit" value="Pay now" />

Ohh i see what the problem is now, you would need to add the code below in your theme functions.php file

add_action( "adverts_form_load", "form_bank_transfer_config" );
function form_bank_transfer_config( $form ) {
    if( adverts_request( "page" ) != "adverts-extensions" || adverts_request( "module" ) != "bank-transfer" ) {
        return $form;
    }
    foreach( $form["field"] as $key => $field ) {
        if( $field["name"] == "custom_text" ) {
            $form["field"][$key]["mode"] = "tinymce-full";
        }
    }
    return $form;
}

It will enable full tinyMCE editor for the custom text field, you will then be able to paste HTML for your form.