Dear stata listers

I'm using stata to analyse time-to-event data. Specifically, I'm trying to relate licensing data to merger and acquisition events in the bio-pharmaceutical industry.
To that aim, I use the Cox PH model, and I've managed to set up and analyse the data in stata (v 14.2) by means of the commands listed below - an excerpt of the datafile can be found in attachment.
The problem I'm facing is related to the analysis of a certain covariate, i.e. the total asset value of the licensees. This value is not constant and is dependent on the licensee.
My question is now: how to correctly implement this specific covariate in the existing Cox PH model.
I've searched in the internet but I haven't found a solution to this particular problem.

Any help/suggestions would be highly appreciated.

Many thanks in advance

Alex (stata 14.2) [ATTACH]temp_12751_1544367960848_705[/ATTACH]

PS In the dataset, I've already defined an additional ID (value) for each unique licensee (named: Licensee ID), the total asset value of the licensee is in a column named Licensee Total Assets (th USD).


*Define variables for Survival Analysis
global time Date
global event M_A
global id ID
global xlist LicenseCumulAll
gen PeriodStart = mdy(1, 1, 2000)

* Set data as survival time
stset $time, id($id) failure($event) origin(time PeriodStart) scale(365.25)

* Cox proportional hazard model
stcox $xlist, cluster(ID)