Can't log into USPS website with sendkeys

I'm writing a C# scraper so I can download my delivery receipts. I'm trying to use findelement and sendkeys/click to login to the usps website https://reg.usps.com/entreg/LoginAction_input . When I attempt this method, I get an invalid login error. I know the code works because I login other sites using the same method or even through JAVASCRIPTEXECUTOR. In my current workaround, I have to TAB over to the input field and call inputsimulator. However, I want to make my application headless which will not play well with InputSimulator as it as not tied to the selenium driver browser so I would really like to use sendkeys or another alternative. I have also tried to use javascript executor without any luck. Please help. Thank you.