The ODVA CIP Standard defines several encodings for strings. A CIP STRING has 2 bytes to indicate length, and then 1 byte per character. A CIP SHORT_STRING has 1 byte to indicate length and then 1 byte per character. In your screenshot this is the byte containing 0E.

So you could try changing the UINT of your STRING64 definition to a USINT, and then rename it to SHORT_STRING.

Whether to use a STRING or SHORT_STRING should be defined by your end device.

Caveat 1:
Some systems always put gaps between variables, so every variable starts on a memory address divisible by 2, 4, or 8.

Caveat 2:
Codesys strings are stroed in memory as a series of bytes followed by a terminating character, 0x00. If you were to call this code again, and this time get a string of only 3 characters (e.g. 'new'), you would end up with a string displayed in codesys like:
'new 20-24-100D'

So you need to now have a 0x00 byte at byte 4. You can do this 2 ways:

1. Clear the memory before you call the function, so it is all zeros. easiest way ould be to declare a

VAR CONSTANT 
    blankSHORT_STRING : SHORT_STRING; 
END_VAR

and then do an assign:

ssHostnameToGet := blankSHORT_STRING;

1. Just write a zero after you call the function, to the index of the length.
   Now you can access the individual bytes of a string, first byte is myString[0], second byte is myString[1]
   so you could do:

terminateAt := MIN(63,ssHostnameToGet.usiLength);
// maybe some alarm, if ssHostnameToGet.usiLength > 63
ssHostnameToGet.sString[terminateAt] := 16#00;