[registry] @="SomeString666"

something.reg :

[HKEY_LOCAL_MACHINE\SOFTWARE\MyApp]
@="SomeString123"

How do i do this in NSIS?

Use the WriteRegStr command, the parameters can be found in the users manual.

Yes, but that's for value's that have a key_name. This one doesn't (and that's why I ask it). When adding above .reg to the registry, it's effect is dat the 'standard' REG_SZ value of a subkey gets the value "SomeString123". Now, shoot me, but I don't see how I do this in NSIS.

Use "" for the key_name field, like this:

WriteRegStr HKLM "Software\MyApp" "" "SomeString123"

Thanks!