403 Forbidden Error when connecting Marketo from Workfront Fusion using custom API module

We are trying to connect Marketo instance from Workfront fusion using custom API module and seeing 403 as a response after running the scenario from fusion. Other modules of Marketo  in fusion working as expected like create /update/list record etc,  only issue is with custom API

Has anyone faced similar issue and found a solution ? Kindly share details.

Thanks,

Anu