macro for save as from .xls to .mthml

I would like to create a click button on Userform that allows user to do "save as" the data sheet with certain cells and in .mhtml format.

I have tried code shown below but it's not working. please if anybody could help for this. Thanks.

Public Sub SaveAs()
Const PATH As String = "c:\"
With ActiveWorkbook
'Range of the cells that I want to export
Range("K12:K3054").Select
.SaveAs FileName:=PATH & _
'file name and type of file
.Sheets("Sheet1").Range("A1").Value & ".mthml"
End With
End Sub

SaveAs is a method of Workbook an Worksheet, but not Range. You cannot save a range as a file.

Also, Excel does not provide native support for MHTML. You can specify a format of HTML.

I would like to ask a little bit more. Now I try a new code (shown below) to export the file as .html. My problem is the data is not exported properly. Web browser can't read it and I need the export file to contain formatting and colors as shown in excel. If it's possible?

Sub Button1_Click()
Const PATH As String = "c:\"
With ActiveWorkbook
.SaveAs Filename:=PATH & _
.Sheets("sheet1").Range("AE3").Value & ".html"
End With
End Sub

It would serve you to record yourself doing an actual SAVEAS HTML and then work with adapting the resulting code. Saving to HTML is an entirely different syntax.

Sub Button1_Click()
Dim fName As String, fPath As String

fPath = "C:\"
fName = Sheets("Sheet1").Range("AE3").Value

    With ActiveWorkbook.PublishObjects.Add(xlSourceSheet, _
        fPath & fName & ".html", _
        ActiveSheet, "", xlHtmlStatic, fName, fName)
        .Publish (True)
        .AutoRepublish = False
    End With
End Sub

That should get you started experimenting with the various parameters.