Initial margin requirements for Spot FX positions are 1% of the total position value.

0.5% of Primary currency + 0.5% of secondary currency.

You are long 100,000 USD/CHF at 1.2688

Your USD margin is 100,000 x 0.005 = 500 USD

Your CHF margin is 100,000 x 1.2668 x 0.005 = 633.40 CHF

Your total margin requirement expressed in USD is 500 + (633.40 / 1.2668) = USD 1000

You will need to have ledger balances in both the primary and secondary currencies to the value of that currency's margin requirement to avoid being charged interest.