Submission in static page

it’s just a page where registered and logged in users can submit their participation

A booking (and the booking from) is always linked to a specific event. You can not say “Hey some day I will be there”. If you do not book a specific event there is absolutely no way of knowing when they will attend.

-How can I automatically add users to an event without the need to log-in to their profile and submit the event for them?

You don’t! Loggin in is the correct and only way to link a user to a booking. Not logging in means they are not verified and anyone could book in their name.
Not logging in = no user linking. Simple.

In Events > Settings > Formatting> Events List Page you can define your 100% custom layout. All you need is basic knowledge of HTML and CSS.

A booking (and the booking from) is always linked to a specific event. You can not say “Hey some day I will be there”. If you do not book a specific event there is absolutely no way of knowing when they will attend.

I was meaning that i don’t want to show the event in the events list. Maybe with some conditional?
The fact that this event isn’t really correlate with a date and a place is just because it isn’t.
It’s like a “subscribe” button, without any consequence to the users.
This way I can manage in the back-end who is really an official member.

You don’t! Loggin in is the correct and only way to link a user to a booking. Not logging in means they are not verified and anyone could book in their name.
Not logging in = no user linking. Simple.

As stated before, i don’t know why (if I’m an Administrator) I can log-in in the front-end as an user and link it to a booking, but I can’t simply do that in my back-end in the Events > Bookings page with the simplicity of a button to integrate users in an event.

In Events > Settings > Formatting> Events List Page you can define your 100% custom layout. All you need is basic knowledge of HTML and CSS.

Done, thank you

I can log-in in the front-end as an user

The key word here is: log-in. If not logged-in, there is no link to any user account. ??

I can’t simply do that in my back-end

Yes you can, but that is EM Pro feature (one of the many great ones). Then you can simply select a user from a drop down and add a booking using the integrated Offline Payment Gateway, without switching users.
https://paste.pics/b4ad96bdc7de81cd56dcc21234ee2e4a

This way I can manage in the back-end who is really an official member.

Sounds like you “need” BuddyPress, which is fully integrated into EM. I have not used it myself, but I do see a lot of other EM users (here in the forum and Pro forum) using it. That way you can assign events to certain groups, allowing only those group members to book a ticket for that event.

It comes across as if you want to do a lot of work by hand yourself. (No rudeness intended!). Perhaps if you described what your ideal, automated booking situation would have to look like, we can point out some helpful settings and/or plugins. ??