how to get an output to a variable (@incode) from exec (query) command

CREATE proc  [dbo].[pGetCode]--[pGetCode] 'InvestmentGroup', 'Code', 'I'
(

@TableName  varchar(20),
@ColumnName varchar(20),
@key varchar(10),
--@TotalDigits varchar(10),
@finalcode varchar(20)=null output 
)
as
begin
--create table #temp (incode varchar(20));
declare @incode varchar(20)='';
declare @a varchar(10)=null;
declare @length int = 1;
declare @string varchar(20) ='';
declare @startposition varchar(10);
declare @query varchar(200);
declare @TotalDigits varchar(20);
declare @rc varchar(100)
declare @OutputParameter varchar(100)
set @TotalDigits = (select character_maximum_length from information_schema.columns  where table_name = @TableName and COLUMN_NAME = @ColumnName)
set @startposition=Len(@key)+1;
declare @ss varchar(max)

set @query='declare @incode varchar(20) select @incode = SUBSTRING( max('+@ColumnName+') ,'+@startposition+','+@TotalDigits+') from  '+ @TableName +'  where '+@ColumnName+' like '''+ @key +'0%'''

--@incode = exec (@query)    -----IS THIS POSSIBLE  ,  HOW TO GET THIS querys Output to @incode variable

select @incode