Yes, that is not allowed. Write the code as follows:

TEXT* inkey_chatmsg_text =
{
pos_x = 10;
layer = 1;
strings = 2;
string = "Nachricht:",chat_input;
}

function main() {
video_mode = ...;
...
After the first frame, screen_size is set by video_mode and you can use it:
wait(1);
inkey_chatmsg_text.pos_y = screen_size.y-20;
}