(MX 04 Pro) Focus a text field

This should be easy enough. I need to select an input text box on keypress, as the mouse will be busy. Ive had a look and found setfocus, but everything i try tells me There is no method with the name ‘setfocus’. So somethin along the lines of

on (keyPress “<Left>”){
myinputbox.setFocus();
}

or whatever. Thanks in advance.