I've noticed that when navigating to a B.I. Portal page, the tab that was last viewed in a previous session is the one that is displayed in the new session. Is there a URL that specifies which tab is the active tab when navigating to a B.I. Portal page?

You my friend must have a role assigned to you that has permission for this. As an administrator in my environments, my portals behave this way, but as a Basic User, they always get the defaulted tab that is set first when you save from the portal editor.

Go into the security center, and click on Roles then edit roles, you can see what roles have what permissions.

So, I, as an Admin, cannot send a Basic User a URL that specifies the active BIP tab?

I do not know if you can send a link like

..BIP/PAGE?&TAB3

But, I know that you can set the page to always open on say tab 3. I believe to do that you would edit the page, click that tab and then save it (i think). I believe you can also give permissions for the user to chose which tab it opens on. I think there is a role that will re-open on the last tab you left, or even a role that will let you customize your own page for what the default tab is. But as far as, having tab 1 the default value and sending a link that says to open it up on tab 3, that might require an NFR.