javascript ( if/else) for dropdownbox witn "custom input allowed"

Hello, I've created a dropdown box and clicked "custom input allowed". Each input value has an export value assigned, which then appears in a separat textfield. That works, if I select the input value via dropdown. If I write in manually nothing happens. Here the same export value should appear in the textfield. And if the customer makes an input which doesn't exist, a window should open "Check your details". Can someone help me, how and especially where I can enter this script? In the attachment my example (unfortunately you can not upload it as a pdf). Thank you in advance!