[R-meta] standard error in predictive nonlinear meta-regression model

I am using a nonlinear meta-regression of the form y ~ p1 * exp(-p2*A), with the following coefficients: 

     estimate     se  zval    pval
p1     5.5447 4.2592 1.302 0.19298
p2     0.3856 0.0835 4.616 0.00000
tau2   0.0000     NA    NA      NA

I want to use this model to predict the effect size (in %) of a treatment, influenced by the variable A. Let's assume, for example, this is the effectiveness of a cancer treatment at a certain age A. For example, when A= 9, effect size is ~19%:

make_pct <- function(x) (exp(x) - 1) * 100

make_pct(5.5447 * exp (-0.3856 * 9))

[1] 18.82309

The problem is that now I don't know how to calculate the standard error at e.g. A=9 using this model. I am sure this is very basic but I don't have any experience in this particular matter. Thanks