The BP Effect for a long calendar spread is the net cost of the spread (cost of the long options - the credit of the short options).  

*Note, the short option (s) must expire before the long option(s).  

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.

Buy to open 4 Jun 43 Calls at $3.00  
Sell to open 4 Mar 43 Calls at $1.00  
The BP Effect of this position is $800 [($3.00 - $1.00) x 4 contacts].