A forward price comes from carrying the asset to delivery.
Borrow, buy, deliver
To set the forward price, borrow $100.00 at 5%, buy the asset now, and deliver it at maturity T equals 1.
F0=S0(1+r)T
Fair forward price
At maturity, the loan repayment is $105.00, so the fair forward for this setup is $105.00. A different forward price would leave one side a riskless arbitrage.
S0=$100.00,r=5%,T=1,F0=$105.00
Model note
This argument assumes borrowing and lending at one rate, no storage costs or trading frictions, and an asset held to delivery.
one rate, no storage, no frictions