Unable to insert membership custom field in e-mail using Smarty
It depends on which message template you are trying to add token to and method you are using for sending email. 

Not all method of sending email renders membership token. If you are successful to render {membership.id} or if you can get membership from smarty then you can use CiviCRM api to render the custom field value for membership based on membership id. 

For ref: How to create tokens for custom fields (participant, individual, organisation)

HTH

Pradeep