Validation: Custom Formula in Acrobat PDF Tag

Dear all,

I know I can define a field validation via regex by using this tag as the field's name in Acrobat:

MyField_es_:custom(regexp="__myregex__",msg="My error message")

How can I do the same for validation type "Custom - Formula"? I can't find anything in the Text Tag Documentation or Knowledge Base.

I just tried

MyField_es_:custom(formula="__myformula__",msg="My error message")

into the blue, but this doesn't work.

Does anyone know how to achieve this?