You are going to have to go back to where you got the data from and ask there - it's not obvious what encoding was used to generate that string.
It looks like Base64, but that decodes to an 18 byte value:
66 01 0A A7 04 1B 36 CC 00 00 01 60 7F 64 78 BA 0B C0
Based on one sample of data, with no idea what the actual binary data should be it's not possible to be sure what to do with the value (GUIDs do have a format, but there are 5 different versions and without being able to "decode" the input first, we can't even tell which one it is!)