What you are doing and asking about is s very common thing. You just have to make sure you are calling the objects of the fomr correctly.
There are lots of examples of how to do this all over the web.
There are direct examples in the PoSH built-in help files ...
# Get parameters, examples, full and Online help for a cmdlet or function
(Get-Command -Name Invoke-WebRequest).Parameters
Get-help -Name Invoke-WebRequest -Examples
Get-help -Name Invoke-WebRequest -Full
Get-help -Name Invoke-WebRequest -Online
... and sample scripts provide via the MS PowerShellGallery.com and the Scripting Guys blog.
Automate login to website
A script to log in to a website, parse the contents into objects, click some buttons and tick some checkboxes.