Is there any way I can indicate on my application that I will be using a cosigner? A cosigner for an apartment takes on the financial responsibility for a lease.Learn how having a cosigner can help someone get approved for an apartment. Adding a cosigner is pretty straightforward with these steps: Do your research: Treat the apartment cosigner as an additional tenant. Make sure the cosigner also fills out a rental application. 3. Establish a Cosigner's Agreement. Adding a cosigner to your standard lease agreement isn't difficult. Rather than writing a new lease entirely, you can simply add an addendum. You fill out an application, present it to the prospective landlord and after reviewing your app, credit report, etc. A cosigner can give landlords the peace of mind they need when approving tenants who may not have a good credit score or renting history.