How can I configure the website to open the login popup as soon as it is accessed?
admin - settings - login required set to enabled.
if you want more granular controls or a different approach, there is also @Don’s guest gate theme component:
does enabling admin-settings-login_required not work for you? i just tested it on my forum and it worked.
so you want it to skip that screen and go right to the sign-up modal? even if they don’t already have an account?
Right, exactly like that. Can you show me how to do it?
no because that would require some customization of your site and i don’t know of any particular code workaround for the login modal. i suspect it can be done but you may need to post in marketplace if someone else can’t help you here.
Thank you for your assistance
This seems like a continuation of your similar question so I’ve merged it together. 
There is the [YourSite]/login link you can use to direct people to (also the [YourSite]/signup for the create account one).
I think the Guest Gate theme component mentioned above would also be a workable option.
Hello,
I’ve made a theme component which is open the signup modal automatically on the first page load per session. You can install it here. 
This is indeed the solution I’ve been looking for. Thank you very much