How do I format the output from OBJECT_DEFINITION so that it looks the same as sp_helptext?

I had similar problem. First turn on this option of SSMS - it is turned off by default:

SSMS/Tools/Options/Query results/SQL Server/Results to grid/Retain CR/LF on copy or save

Close SSMS, and run it again to get it work.

I use this query to get the full text of object definitions:

create view v_get_text_of_objects as

select top 100000
  [name]
, definition = replace(replace(object_definition(object_id),'create view','alter view'),'create proc','alter proc')
, len = len(object_definition(object_id)) 
, type
, type_desc
, create_date   
, modify_date
from  sys.objects
where object_definition(object_id) like '%your text to find in sp%'
order by type, create_date desc

I like to display LEN of the sp text, so that I am sure it is not cropped.

sp_helptext 'sp_helptext'
    --If carriage return found  
    IF @CurrentPos != 0  
    begin  
        /*If new value for @Lines length will be > then the  
        **set length then insert current contents of @line  
        **and proceed.  
        */  
        while (isnull(LEN(@Line),0) + @BlankSpaceAdded + @CurrentPos-@BasePos + @LFCR) > @DefinedLength  
        begin  
            select @AddOnLen = @DefinedLength-(isnull(LEN(@Line),0) + @BlankSpaceAdded)  
            INSERT #CommentText VALUES  
            ( @LineId,  
              isnull(@Line, N'') + isnull(SUBSTRING(@SyscomText, @BasePos, @AddOnLen), N''))  
            select @Line = NULL, @LineId = @LineId + 1,  
                   @BasePos = @BasePos + @AddOnLen, @BlankSpaceAdded = 0  
        end  
        select @Line    = isnull(@Line, N'') + isnull(SUBSTRING(@SyscomText, @BasePos, @CurrentPos-@BasePos + @LFCR), N'')  
        select @BasePos = @CurrentPos+2  
        INSERT #CommentText VALUES( @LineId, @Line )  
        select @LineId = @LineId + 1  
        select @Line = NULL  
    end  

K, so this is how sp_helptext does it. Wrap the results are a table function returning rows for CRLF characters returned by the object_definition.

Since you're using SQL Server 2016, you can use STRING_SPLIT and a trick from Aaron Bertrand to split on carriage-return/line-feed.

DECLARE 
  @olddelim nvarchar(32) = char(13) + Char(10),
  @newdelim nchar(1)     = NCHAR(9999); -- pencil (✏)

SELECT * FROM STRING_SPLIT(REPLACE(OBJECT_DEFINITION(object_id('usp_radhe')), @olddelim, @newdelim), @newdelim);