Convert Dynmaic SQL from MS SQL procedure to PostgreSQL PL/pgsql function

I have some problems on MS SQL procedure convert to PostgreSQL PL/pgsql function. Please give me some advice.  Can I
dynmaicdeclare cursor out of DECLARATION block and in STATEMENTS BLOCK? 

I have many procedure in MS SQL which run dynmaic SQL Example:

if (UPPER(@DBTableName) = 'OT_APPLNCUSTOMER' or UPPER(@DBTableName) = 'OT_APPLTADDRTBL' )

  begin
    Set @SQLStm = 'Declare Cur_Temp_Attribute Cursor  ' +
                  'For select ' + @DBTableField + ' From ' +
                  @DBTableName   +
                  ' Where APPln_Ref_No = ' + '''' + @Appln_Ref_No + ''''
  end
else
  begin
    Set @SQLStm = 'Declare Cur_Temp_Attribute Cursor  ' +
                  'For select ' + @DBTableField + ' From ' +
                  @DBTableName   +
                  ' Where identity_no = ' + '''' + @identity_no + ''''

  end
exec(@SQLstm)
Open Cur_Temp_Attribute

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Thank you very much.