That depends on the login method used by the web site you are accessing. It may be your only option is to pop-open the login page in a Show Web Page action, but it could well be that if you then close it to progress to the next step in the shortcut that it loses the login details.
Some web sites allow for OAuth and there are roll-your own solutions for that if you search this forum.
Some web sites offer an API to get data, and this is the ideal approach for automation as the whole thing is intended to be used for hands-off automated access. Accessing web pages with web page logins is not designed for automation and is one of the reasons you can run into challenges like this. If it is ancestry.com that is the site you are trying to access, then it would be mixed news on this front. They do have an API, but they apparently don’t make it accessible publicly, only to partners.