The margin requirement for a short calendar spread is the cost of the long option plus the margin required on the short option. There is no relief on calendar spreads when the short option expires after the long option.
Additionally, tastyworks does not recognize calendar spreads in cash-settled indexes. Any short option without a corresponding long option in the same expiration cycle will be treated as uncovered.
With the underlying at $40:
Buy to open 2 Mar 43 Calls at $3.50
Sell to open 2 Jun 43 Calls at $4.00
The margin requirement on this position is $2,900 ([(.25 x 40) - 3 + 4] + 3.50 ) x 2.
The margin requirement on the short call is: [(.25 x 40) - 3 + 4] x 2 = $2,200
The margin requirement on the long call is the cost of the option x the number of contracts: $350 x 2 = $700