Hi there,
You have to apply for an Off Campus Work Permit (OCWP) using the below sentence on the CIC website search box:
Application to Apply for a Work Permit - Student Guide
IMM5257 seems incorrect. The correct form is IMM5710. You first have to apply online through CIC, which will contact your DIR at the institute of you are in good standings. DIR will issue an EVN which you have to use in the IM5710 form.
Good luck!