I am trying to auto updated choice (dropdown) field depending on single line textbox that is filled when the Nintex form loads once the dropdown option is loaded for the box to get greyed out

When form loads

Column 1 called "connection" (single line textbox) = "Dual" 

Column 2 called "track" (choice field dropdown) = "secondary"

Client ID JavaScript variable name for each control is :

DualJSID

secondaryJSID

In column 2 there are more choices but when the form loads with only Dual in column 1 l want column 2 to have "secondary" chosen and the column 2 then greyed out 

Any help would be greatly appreciated

I know this can be done via javascript but that is what l need help on