Creating Windows login/user for Mixed Always on Environment+ Instance without Always ON
My understanding is that you are not approaching this in the right way. you just copy the login using the same sid to all the secondary server. Users get replicated since db is part of AG.

Basically you want to create/replicate logins that are created on primary server to all secondary servers in an AlwaysON availablity group topology.

You can just use dbatools Sync-DbaAvailabilityGroup that will take care of 


  Syncs depdendent objects for availability groups. Such objects include:


SpConfigure
CustomErrors
Credentials
DatabaseMail
LinkedServers
Logins
LoginPermissions
SystemTriggers
DatabaseOwner
AgentCategory
AgentOperator
AgentAlert
AgentProxy
AgentSchedule
AgentJob



  Note that any of these can be excluded. 


Edit : Below script will help you :

SET NOCOUNT ON    
DECLARE @user_to_be_added sysname    
DECLARE @sqltext nvarchar(max) = N''  

-- change below to your user to be added .. make sure that the login is already existing
SET @user_to_be_added = 'Domain/Login'  
SELECT  @sqltext += char(10)+ 'use '+quotename(name)+';'+char(10)+ 'CREATE USER '+quotename(@user_to_be_added)+' FOR LOGIN '+quotename(@user_to_be_added)    
FROM sys.dm_hadr_availability_replica_states ars
inner join sys.databases dbs ON ars.replica_id = dbs.replica_id
 where dbs.state_desc ='ONLINE' AND databaseproperty(dbs.name,'isReadOnly') = 0  
and ars.role_desc = 'PRIMARY' -- we want to filter only Primary AG where dbs are writable.

 if  (@sqltext &lt;&gt; '')
 begin
  select @sqltext 
  end
  else 
    begin 
    print 'There is nothing to print - @sqltext is blank'
    end