Welcome. To summarise this post - if you just want to get a few documents (filings) then using the Companies House Find and Update company website to search is simplest. Alternatively you may wish to register for API access (e.g. programmatic access to data). If you wanted a large amount of data then the “Bulk Data products” are the way to go.
It looks like you’re trying to use a garbled version of using the credentials in the URL to pass a username and password for http authorization. This is generally not recommended e.g. see the “Access using credentials in the URL” section in the article on http Basic from Mozilla.
Firstly - if you wish to use the API you will need to register to use this. It is free but you do need to register to obtain an “API key” (note - confusingly this is the “username” part of the http Basic authorisation - in use the “password” part is blank).
If you didn’t already I recommend reading through Companies House documentation. I’d also recommend the information pages made by another forum user - chguide.co.uk. For downloading filing documents I recommend reading some of the detailed posts on this forum as this sometimes causes people trouble. You can search for these using the search icon at the top right.
Your third post asks about the difference between using the API and using the URL for the Companies House website directly. I don’t know the exact differences but if you are planning to access things programmatically then using the API is the way to do it. There is no guarantee that the form of the URLs on the web site won’t be changed with no notice at all - or the format of the data received will stay the same. The API can of course change but there should be some notice on this forum! It is also possible that the web site may have different rate limiting or detect “unusual access” and block your IP address if the system thinks you are attempting to access other than expected.
Good luck (and read the documentation!)