forecast with ARDL model , AR(q) et VAR(q)

[jerry mental]

hello, i need help

i need to forecast the GDP in the next 3 years with 3 models (ARDL, AR, VAR) i don't how to do in Stata.
i do this but it isn't work:

reg dgdp L(1/2).dgdp dinvest
tsappend, add(12)
estimates store adl
forecast create adl, replace
forecast estimates adl
forecast solve, log(off)

i will really appreciate if someone help me, i have to give my homework on sunday and i have block in this step. i would like to know either what command can i use to know how many lags
that i have to schoose on all my regression.

[Sebastian Kripfganz]

Regarding the optimal lag selection in an ARDL model, the following topic might be of interest for you:
ARDL in Stata

For the AR and VAR model, please have a look at the varsoc command.

[jerry mental]

i read the topic, but i didn't understand when you talk about the lag on the ARDL model. All the variables in the model don't need to have stationary?