Creatives can be submitted during the Campaign creation flow or added later.
If you have unpaid Campaigns or in Draft status, you can edit them and add more Creatives.
Select your campaign that has no creatives or has a draft
Once your campaign is open navigate to the "Creatives" tab then select "+New Creative" highlighted in the below screenshot.
You can now select whether you want to select "Upload your Video Creative" or "Generate your own Video ad"
If you have paid for your Campaigns, you can go to the LIVE or PENDING campaign then select it and follow the same above steps to add more Creatives.
Once submitted your new creative will then go through the Creative review process