Let \(X \sim \text{TPXG}(\alpha,\theta)\). Then the cumulative distribution function of X is given by:
$$
F(x)=1-\frac{(\alpha+\theta+\alpha \theta x+\frac{1}{2}\alpha \theta^2 x^2)}{(\alpha+\theta)}e^{-\theta x}
\quad x,\theta,\alpha > 0
$$
References
"Sen, S., Chandra, N. and Maiti, S. S. (2018). On properties and applications of a two-parameter XGamma distribution. Journal of Statistical Theory and Applications, 17(4): 674--685."