If you are in the US and you want to do margin trading as a retail trader, forget about it. It is much easier getting out of the US, getting a citizenship and a less strict country and then carrying out margin trading from there
On a serious note, the reason almost zero platforms offer the service to US citizens. This is because CFTC keeps restricting, suing and fining them. If there's any exchange still doing so, it's probably still brand new, less known and out of the radar from CFTC.
Kraken say you need to have more than $10 million in assets before you think about Margin trading in their platform
Do you have more than $10 million in assets, OP?
For individual clients -- You are required to have more than $10 million in assets invested on a discretionary basis. In general, this means assets (for example, cryptocurrencies, stocks, bonds, mutual funds, real estate investments, cash held for investment purposes) for which you make the decision to trade or invest, as opposed to assets that another person trades or invests on your behalf. The $10 million in assets do not include your personal residence or other assets you own that are not part of your trading or investment portfolio.
For institutional clients -- The institution you represent must have more than $10 million in total assets. This test focuses on gross assets, as opposed to net assets (assets minus liabilities). If you represent an institution and the institution doesn’t qualify as an ECP under the $10 million in total assets test, your institution may qualify as an ECP under one of the other categories of the ECP definition. Please review the categories and contact Kraken support if the institution you represent qualifies for at least one of them.
