Don't use a script, use an auto-enter calculation in Field B:
FieldA = "Multiples" ; "000" ;
FieldA ≠ "Multiples" and Self = "000" ; "" ;
This will prevent the incorrect data from beng entered. It won't give a message: if you need a message then use field-level validation and specify a custom message.
If you use Vaughn's approach you can clear the check box that allows the user to enter the field in Browse mode. The formula will update the field adn the user won't be able to manually change it.
OK, put this in an yes it does the autocomplete of Field B if I select "Multiples", but if I enter any other option in Field A it still allows me to enter "000" into Field B which they should not be able to unless "Multiples" is selected, so almost there...
What should they be able to put in if the value for Field A is not multiples?
You can add validation by formula to the field and have it check the value of Field A.
Any of the other options (in the radio button set) there are currently about a total of 10, 1 of them is "Multiples" in Field A and in Field B it's a drop down list with maybe 50-100 options where 1 of them is "000"
Ok... I think I see what you are getting at. If Field A = "Multiple" then Field B should be "000" if Field A is not "Multiple" then Field B should not be "000" but should have some other value.
So I would do this through the Validation by formula plus a bit.
Attached is an example that I think does what you are asking. I used a combination of an auto enter formula to provide the initial value for FieldB, a validation formula that prevents the user from using an illeagle combnation and an onexit script trigger that commits the record when the user leaves fieldB. This forces the validation formula to kick in.
fieldA.fmp12.zip 9.1 K