Salisbury won the second leg 4-1, after Swindon had won the home leg 1-0 last year, so Salisbury went through to the final with a combined score of 4-2.
Salisbury will now face Chippenham Town in the final in April.
Swindon's goal was scored by Ashan Holgate, and the squad included Christian Roberts. Alan Reeves said: "I expect we will give Christian another chance to increase his fitness. It is chance for us to get a win after Saturday’s disappointment."