Dynamics CRM online plug-in security exception

Hi,

I am new to Dynamics CRM. I created a plug-in used to forward the lead from master organization to child organization.

So I need to connect the organization service of child organization, bu it cause a security exception in online sandbox mode.

But it works fine in on-premises version without sandbox mode.

After some research I think it could caused by the sandbox is partial trust and the DefaultCredentials I used to create connection need full trust.

Is my understanding correct? And is there any way to implement this? 

Thanks