Barney proposes to Robin in Season 8 Episode 12 of How I Met Your Mother with the title “The Final Page – Part Two”
Barney tells Ted that he is going to propose to Patrice and that he should keep it a secret. However Ted feels that he should tell Robin about it because she still has feelings for Barney, but only at the right time. One such moment came at MacLaren’s Pub, however Marshall and Lily interrupts it. After Marshall and Lily leaves Ted asks Robin if she could be his date to the opening of new GNB Headquarters (Ted was the designer of the building). Marshall and Lily would be there but only for 11 minutes as they stick to their 24 hour time schedule of free time from parental responsibilities.
Ted decides to call Marshall to tell him about Barney’s secret and how he asked Robin to be his date. Marshall makes Ted swear not to speak to Robin about it. But he decides to tell her on their way to the GNB opening. Ted asks her if she wants to do anything about it, she answers no but makes a loud outburst after hearing that it would be on the roof of the World Wide News (WWN) building. She says that detail stings but she is not in love with Barney.
After stopping at the WWN building Ted convinces Robin to interrupt Barney and Patrice’s proposal. On opening the door to the roof Robin sees a roof decorated with candles and flower petals. On the ground was a paper for Barney’s Play Book with the title “The Robin” which outlined all the things Barney did to get Robin back. She turns around, sees Barney and starts to speak sadly about Barney’s manipulation. He then told her to turn the paper over. On that side of the paper is says “Step 16: Hope she says yes”. She lowers the paper to look at him and he was on his knees asking her to marry him. She concents with a “yes” and the kiss.