This is usually the letter of support issued by the nominating province.
Given the other details you mentioned in your post - unfortunately you may not be able to obtain this. See below:
As of June 20, 2015:
Due to Federal level changes to the issuance of temporary work permits, as of June 20th, 2015, OINP no longer has the authority to issue support letters for open work permits. This only applies to nominees in streams under the Human Capital Category and includes all streams that do not require a job offer.
OINP Masters Stream is listed as under the Human Capital Category.
.../atb