Add to Plist

I have this plist:

Root (dictionary)
-Samples (array)
–0 (dictionary)
—sample_name string s1
—sample_index string 7
–1
—sample_name string s4
—sample_index string 4
–2
—sample_name string s2
—sample_index string 8

I have tried to add this to it:
–3
—sample_name string s7
—sample_index string 2

I have tried to use defaults write, but can’t find right code to do it:
do shell script "defaults write myscript1 Samples …