HI,
You can follow below steps to add Form on Account Entity.
1. Go to make.powerapps.com
2. Create One Unmanaged Solution
3. Then Click of Add Existing Button to Add "Account" Table.
4. After this, Form Tab will appears then click on Add New Form and select the Required Form Type Such Main, Quick view etc.
Upon Click on Form Type a Form designer screen will open and after adding required components.
You need to publish all the changes.
Now, you will able see the new form on account entity.
Just in case the new form not visible in model driven App then Click on App Designer and check the forms for account entity.
