You need to set up your freelancer aspect within your current Upwork login account. And then you will need to be approved.
There is not a button you can click to guarantee that you can work as a freelancer.
If you would like to be able to submit a proposal as a Freelancer, you will need to create a Freelancer account and get it approved.
You can be a Client and a Freelancer using the same account. You'll use the same account to log in to Upwork, but the two parts of your account will be kept entirely separate. You'll be able to easily switch between the two as needed. To create a Freelancer account on your existing account, please follow up instructions in this help article.
You will need to fill out your application and submit it for a review, please allow up to 24 hours for the review to be completed.