Related Drop down list

You can do this with Conditional Contact Form 7 plugin, for each values in your first dropdown you need to create a corresponding dropdown field,

`[select first-dropdown “chose a value” “value 1” “value 2”]
[group dropdown-value-1]
[select value-1-dropdown “chose option for value 1” “value 1a” “value 1b”]
[/group]
[group dropdown-value-2]
[select value-2-dropdown “chose option for value 2” “value 2a” “value 2b”]
[/group]


You then need to setup your visibility rules, and to showdropdown-value-1` group when ‘value 1’ is selected in the first dropdpwn and so on. See this tutorial which explains all the steps in details.