Calculation field to show a CSS button?

I want to add a link button to a Globimail boilerplate that will send the client a link to a feedback webform from a booking however I need to add the booking ID so when it is submitted it comes back to the booking.

I have this set up as a variable in Globiflow that updates a multi-line text field in the booking to be used as a token in the boilerplate but it doesn't include the booking ID?

"<a style='background-color:#ff0066;border: none; color: white; padding: 12px 16px; text-align: center; font-size: 14px;' href='https://podio.com/webforms/22670403/1602523?fields[booking-id]='". [(Booking) Booking No.] . "'>GIVE FEEDBACK</a>"

This seems to work but without the button styling - I'd rather it be a button. What am I doing wrong?

"<a href=\"https://podio.com/webforms/22659855/1601521?fields[booking-id]=". [(Booking) Booking ID] . "\">Feedback Request</a>"

Any help gratefully received.