Force Keystrokes To Renoise

Kore opens as a plugin i Renoise, and from Kore you can open plugin synths, which open in their own windows. When you open a synth plugin in this way, you can no longer play the Renoise keyboard, because Renoise has now lost focus.

Is there a way to send all keystrokes to Renoise regardless of which application has the focus?

I tried to create this with autohotkey, but it didnt work. Maybe there is some other way?

You mean all keystrokes to Renoise or to the Core plugin?
If you want most keystrokes to Renoise uncheck the “Enable Keyboard” checkbox in the lowerleft frame of the plugin window.

I mean all keystrokes to Renoise. I have enable keyboard disabled, and can therefore play notes from the computer keyboard with one plugin window in focus. But when a third window is opened from within the plugin, no keystrokes reaches Renoise, so I cant play notes.

AFAIK there is nothing we can do about these scenarios with a new separate window within the plugin.
What I do is pressing alt+tab once to move focus back to renoise.

That is just platformwise a limitation…
Keyboard focus stuff that handles input redirection. Nothing that can be corrected from within any particular application.
Even if you would like you use a tool like ScriptIt (that could perform things you desire), it will first focus the window and then send input to it.