Sociedad travel to the Camp Nou this evening having failed to beat Barcelona in their own stadium since a La Liga clash in the 1990-91 campaign.
However, Prieto is confident that the Basque outfit can secure a result ahead of the return leg next week.
"The key is to keep the tie alive for the return game. With the support of our people there are a thousand possibilities. I believe we can get a draw first," he told reporters.
"We know Barca and we how know Tata Martino plays. Competing against them is always difficult but we go with a lot of enthusiasm and a desire to make history."
The first semi-final will be contested between Real Madrid and Atletico Madrid.