If you are an eligible investor, applying for placements through OnMarket is the same as applying for IPOs, with the key difference being the short time frame that the new shares are available.
The amount of time placements are open for applications can be as short as a few hours, although it may be a couple of days. Before applying, make sure you read the details in the T&Cs and the More Info section of each offer. Be aware that the company might close the offer early.
Following the placement, the shares are settled and allotted (i.e. created) and then trade equally with the pre-existing shares of the company.