How do i auto populate a text field from other text field

@ls_rbls  thanks for your quick response. 

 the signature is in a Text box (this.getField("Textfeld13").value = gUserName; ), in my case i need to insert material number in text box (a), then the username will be generated in (b)

How are you fetching your signature? Is it a digital signature field? a custom stamp?

Where is this signature located?

In any case, I would test if the event value of the target field "a" is not a number (NaN) using a custom validation script  like:

event.rc = false;

var mySignature = "Me, my self and I"
var f = event.value/event.value;

if (f !== 1) { 

  this.resetForm(["b"]);
 
  event.rc = true;


} else {

  if (f !== NaN) { 

  event.rc =

  // insert your script here 
  
  this.getField("b").value = mySignature;

 }
}

If the event value is indeed a number, and it is divisible by itself, it will equal 1. So if it is a number, your script will execute and populate field "b" with whatever script you have that generates your signature.

In my example above, I've declared a variable with "Me, my self and I" as an example for any given name to poplate field "b" if the event value is a number divisible by itself to equal 1.

Any other event value that is not a number will produce "NaN" as a result and field "b" will reset and clear.