When the next tab is clicked, store the value of the textfield in a variable.
If writing your ActionScript on the timeline, this is the code:
// On frame 1:
// Create the variable to store the textfield value
textfieldValue:String = "";
function tabClicked(event:MouseEvent):void {
// Store the value of myTextField
textfieldValue = myTextField.text;
}
// On frame 2
// Populate the new instance of myTextField with the stored value
myTextfield.text = textfieldValue;