How can I create online forms
They could include the background check questions in the application form
If the background needs to be a separate form, I would recommend using another tool to build the form specifically for the background, and then link it from the Welcome page as you indicated. Google Forms could be one option (but here are a few: https://zapier.com/learn/forms-surveys/best-online-form-builder-software/ )
Having a printable form that could be filled out, and then uploaded could be another option, but that requires people to scan the document or take a photo of it.