To comply with the ROFR, you submit a waiver request to HGVC on one their forms and a copy of the contract. Even though HGVC does not hold a ROFR on Flamingo, its sometimes done simply to ensure that there are no impediments to the sale. Typically, they can take up to 30 days to decide if they want to buy it at the agreed upon price or not.
If the TS is at an affiliate (some of which are managed by HGVC), you need to check to see if they hold a ROFR and if they do, comply with that resort's notice procedures.