Setting the initial output value for state space block in MATLAB
According to their onine help part of the parameters to be set is an Initial conditions entry in the block, itself (as mentioned by AVK in the comments):

Initial conditions — Initial state vector
0 (default) | scalar | vector
Specify the initial state vector.
Limitations
The initial conditions of this block cannot be inf or NaN.
Programmatic Use
Block Parameter: X0
Type: character vector, string
Values: scalar | vector
Default: '0'