How to use Option Button !

Hi all member !

I design the report with Request Form have 3 OptionButtons
Their caption are: "Choose 1","Choose 2","Choose 3"

I set SourceExpr to my Option variable is myOption.

and define myOption is Option type in C/Al Global.

But when i set OptionValue for Option button, the system said:
"You cannot enter enter 'Choose 1' in Option. The cursor is in front of the invalid character"

So this is my problem:

How and where i must to define value for myOption variable? ( I've defined my variable myOption in C/AL global but I don't know how to define its value? )

Thanks for your advice !