Create a new field that's synced and then add the timetrade/calendly URL to that field. Then you can do
there may be more you want to do to put that URL in there depending on how things are built out in SFDC.
Thanks Josh- I had my ops team created the custom field but I don't see it in Marketo yet - been about 3 hours, and the permissions have been checked. I noticed while poking around that there are quite a few other fields available at the SFDC user level that don't make it to Marketo - what am I missing?