Hide and Show Forms at a Form level; Please help, Thank you!

Hide and Show Forms at a Form level; Please help, Thank you!

Hello
I have a multi - link object on my form.  Based on the value chosen on the form, I would like another form (not objects)  to appear below.
 
Example:
Multi-link name is Activity



Multi-Select Values BDI,HAC
If I choose BDI
 
I want to show a form called BDI_Form
 
If I choose HAC
 
I want to show a form called HAC_Form
 
 
I tried the On User Input feature on my Multi Link but it doesnt work
 
if (input.Activity  ==  "BDI")
{
    show BDI_Form;
    hide HAC_Form;
}
else if (input.Activity  ==  "HAC")
{
    show HAC_Form;
    hide BDI_Form;
}









 
The error message states:
 
Error in statement at line number 1 The Left expression is of type STRINGLIST and the right expression is of type STRING and the operator == is not valid here