<a href="/mytext.html" target="_blank">open mytext</a>
but I need that the click opens a fixed-with window or a pop-up window.
I can't use java script, because the pagefile editor cut off it.
How can I do?
If you want a page resource to open in a new window, you should make that option available in the settings page.
Go to Site administration > Plugins > Activity module > Page.
Change the Available display options to In pop-up.
Now you can go into your page settings and change the page to open in a pop-up.
Hope that is what you wanted.