Solved
Action Button’s AddRecord expects the Reference Field in the format shown here,
"[index_num_1,index_num_2,......]"
This can be achieved by creating a list of the Reference Field and converting it to a String
str(list($Experiments.id))
with the final code looking like:
#action button linked field within Sample table
actions = []
# Adding Samples
actions.append(
["AddRecord", "Sample", None, {
"Experiments": str(list($Experiments.id)), # Reference List , works as expected
"Process_ID": $Process_ID.id, # Reference Field, works as expected
"Tests": $Tests, # Standard Field, works as expected
"Results": $Results,
"Repeats": $Repeats,
}])
return {
"button": "Duplicate {} records".format(len(actions)-1),
"description": 'Duplicate "{}" records from {}'.format( len(actions), $Process_ID.Process_Name),
"actions": actions,
}