You can, but the form is processed on a time scale of weeks, not… tomorrow.
You can – and probably should – submit your domain to the Public Suffix List. Doing so would cause web browsers – and Let’s Encrypt – to treat your domain more like a TLD. For example, customers would be unable to see each other’s HTTP cookies, and Let’s Encrypt would consider each customer separate for domain rate limiting purposes.
However, new Public Suffix List submissions also take some weeks to percolate down to the Let’s Encrypt production rate limiting system.
Edit: I almost forgot. Adding your domain to the Public Suffix List may be impractical if you host some of your own services on subdomains and do need to share things like cookies across them. But, in my opinion, it would be better to reorganize your stuff and still go forward with the PSL.