I would do it as such:
variable charString="0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z"
variable randString=""
Condition:
-On start of layout
-Repeat 25 times (the length of the string to generate)
Action:
-add tokenat(charString,floor(random(25)),",") to randString
To get the char of a certain index in the randString, use expression mid(randString, 0, 1), this will get the 1st char of the randString.
Reference to using tokenat,mid,random:
construct.net/en