Dynamic 365 powerapps can't get access to the GUID to do a Lookup

Hi

My first issue:

I trying to get to the field "new_samplegroupid". That exist on the "Sample Group Detail".  I do not see this field anywhere.

LookUp('Sample Groups', 'Sample Groups', GUID(txt_SearchOption))

Second:

I would like to remove and html page that I have in Dynamic 365 and replace it with a powerapp combo box.

I have a Sample Group entity, Sample Group Detail entity (This is the entity that will get updated) and Sample Entity (The Sample entity is the combo box items.

I need to pass the sample group GUID to the powerapp from Dynamice 365 using javascript (This I can do). 

The powerapp will select the existing items from the "Sample Group Detail" entity and load the data into the DefaultSelectedItems on the combo box. 

I need will need to add the select sample codes to the "Sample Group Detail" entity.

When the "Sample Group Detail" entity is being updated I need to store the "Sample Group" GUID on the "Sample Group Detail" entity because this is a lookup on the "Sample Group Detail" entity.

Also, because the powerapp will be used as a web resource.  What will be the best approach to saving the data?  I do not want to have 2 SAVE buttons on the form.

Here's my Dynamic 365 web resource that I am trying to change to powerapp.