Syncing a new custom object from Salesforce

Hi, We have a Salesforce custom object that is linked to both Leads and Contacts, and we'd like to sync it with Marketo.

At the moment, the object is not visible in Marketo Salesforce Object Sync.

I've reviewed the Salesforce Object Sync setting instructions in the Adobe documentation here: Enable or Disable Custom Object Sync and the post Syncing a new custom object from Salesforce

I have a question and would appreciate any guidance:

• From the related post that said- “It’s not necessary to disable global sync to add a new custom object or refresh the schema. Instead, refresh scheme directly, which should be safe without disabling global sync.”
  How can I refresh schema directly? when I click on “Refresh Schema” I get disable global sync notification pop-up which restricts any further action 

Thanks in advance for any help!