Use shortcut go to insert-object-formula

I use Kub;untu 18.04 and i want when i am to LibreOffice Writer with one shortcut make:
type a math and then go back to Writer to type text

How can do this ?
can anyone Help me?

Go to Tools → Customize → keyboard tab. There, select “Insert” under Category and the fist instance of “Formula” under Function. Now pick an empty keyboard shortcut from the above list (like Ctrl F) and press Modify

Thank you!