Starting in version 3.5.1 of the SDK, it no longer allows you to create a custom blueprint ID, as it did in versions <=3.5.0.
Even when using a valid UUID with the avtr_ prefix, that no one else has taken, it gives the following error:
[Always] Attempted to load the data for an avatar we do not own, clearing blueprint ID
UnityEngine.Debug:LogError (object,UnityEngine.Object)
VRC.Core.Logger:LogError (string,VRC.Core.DebugLevel,UnityEngine.Object)
VRC.SDK3A.Editor.VRCSdkControlPanelAvatarBuilder/<HandleAvatarSwitch>d__86:MoveNext () (at ./Packages/com.vrchat.avatars/Editor/VRCSDK/SDK3A/VRCSdkControlPanelAvatarBuilder.cs:1330)
System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<VRC.SDKBase.Editor.Api.VRCAvatar>:SetException (System.Exception)
VRC.SDKBase.Editor.Api.VRCApi/<GetAvatar>d__31:MoveNext () (at ./Packages/com.vrchat.base/Editor/VRCSDK/Dependencies/VRChat/API/VRCApi.cs:573)
System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<VRC.SDKBase.Editor.Api.VRCAvatar>:SetException (System.Exception)
VRC.SDKBase.Editor.Api.VRCApi/<Get>d__17`1<VRC.SDKBase.Editor.Api.VRCAvatar>:MoveNext () (at ./Packages/com.vrchat.base/Editor/VRCSDK/Dependencies/VRChat/API/VRCApi.cs:378)
System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<VRC.SDKBase.Editor.Api.VRCAvatar>:SetException (System.Exception)
VRC.SDKBase.Editor.Api.VRCApi/<MakeRequest>d__16`2<object, VRC.SDKBase.Editor.Api.VRCAvatar>:MoveNext () (at ./Packages/com.vrchat.base/Editor/VRCSDK/Dependencies/VRChat/API/VRCApi.cs:341)
System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<System.ValueTuple`2<VRC.SDKBase.Editor.Api.VRCAvatar, System.Net.Http.HttpResponseMessage>>:SetException (System.Exception)
VRC.SDKBase.Editor.Api.VRCApi/<MakeRequestWithResponse>d__15`2<object, VRC.SDKBase.Editor.Api.VRCAvatar>:MoveNext () (at ./Packages/com.vrchat.base/Editor/VRCSDK/Dependencies/VRChat/API/VRCApi.cs:332)
UnityEngine.UnitySynchronizationContext:ExecuteTasks ()