Remote backup to folder path in domain
You may proceed with something like creating a proxy account and then assigning the same to SQL Agent job:


Assuming you have Maintenance plan created, which will have its corresponding job created out there on Job activity. Create a SQL login which later you need to give the required roles to execute the SQL agent jobs and assign this login as job owner of the Main'ce plan job
Before you create a proxy account you will need to create a credential as explained in How to: Create a Credential 
Now for the SQL login created earlier assign the roles out of below as per you're security standards:


SQLAgentOperatorRole
SQLAgentReaderRole
SQLAgentUserRole

Once login has been created with sufficient roles as above you can
create proxy account as explained How to: Create a Proxy. In the ‘Principals’ option, select the SQL login which was created in above step.
Now the final step would be to run this proxy account in the job step
properties of the maintenance plan. You can select the proxy created from dropdown under the section which says "Run As".
Finally test the proxy by executing the job.