This code can be copied and pasted into Groovy Script test step and executed
result = com.eviware.soapui.support.UISupport.prompt("Please select a payment type", "Payment Type", ['CreditCard', 'BankAccount'])
Note: The first quote section is the note above the dropdown box. The second quote is top bar of the popup.
A Step Further
Normally we would want to capture this value and put it somewhere such as a property. You also can just point a test step tag by the Get Data feature and point to the Groovy Script ‘Result’.
Lets add the next line of code to transfer the result to the testSuite.
testRunner.testCase.testSuite.setPropertyValue( "paymentType", result )
Note: This will write to testSuite this Groovy Script is in