Enable Bottom

I have created a userform in which
I would like that the “Ok” bottom is enable only if
all text boxes are complete
(so there is a string inside)
but I have not idea what instructions
to use.
Can you help me?
Thank you in advance

Base ? Calc ? LO version ? Operating system release ?

Sorry Rautamiekka,
Calc Windows 7 and Windows 10

Thank you