已知函数$f\left(x\right)=A\sin \left(x+\frac{\pi }{4}\right)$，$x\in R$，且$f\left(\frac{5}{12}\pi \right)=\frac{3}{2}$.
（1）求$A$的值；
（2）若$f\left(\theta \right)+f\left(-\theta \right)=\frac{3}{2}$，$\theta \in \left(0,\frac{\pi }{2}\right)$，求$f\left(\frac{3}{4}\pi -\theta \right)$.