How to append GUID data from Pro to Arc Online?

Question

I need to append data that's stored in a file geodatabase to a web feature layer. I have created a GUID on the web feature layer, but when I try to append the file geodatabase's foreign key to the web layer I get the following:

Failed on input OID 3922, could not write value '{028C6C51-9C71-4D14-8E4C-0FA1B5B49A6F}' to output field preserve_guid

Additionally, when I try to use the admin "services" to "update" the field, I notice:

"name" : "preserve_guid", 
 "type" : "esriFieldTypeGUID", 
 "alias" : "Preserve GUID", 
 "sqlType" : "sqlTypeOther", 
"length" : 0, 
 "nullable" : true, 
 "editable" : true, 
 "visible" : true, 
 "domain" : null, 
 "defaultValue" : null

I don't know if length matters for GUID, but it is 38 for the other GUID types in my layer. Okay, I'll just update the length to 38. Here's the same fields after "successfully" "updating":

"name" : "preserve_guid", 
 "type" : "esriFieldTypeGUID", 
 "alias" : "Preserve GUID", 
 "sqlType" : "sqlTypeOther", 
"length" : 0, 
 "nullable" : true, 
 "editable" : true, 
 "visible" : true, 
 "domain" : null, 
 "defaultValue" : null

What is going on?

WillHouston · Answer

Well, I was never able to append those guids, but I appended them as text and my script was able to match the relationships anyway.

*Edit: There was another issue with copying those guids. I fixed it yesterday and might mix up the details, but the primary key (GlobalID) was not appending properly to the text field in ArcOnline. I copied those values to a different GUID field within ArcPro and was able to append that to the text field in AGOL.