I know that this should be a very easy issue, but the way actually adopted in Microsoft Edge is, let me say, “different” from the one we are habit to use in other browsers like Internet Explorer … and may be you can find not easy to find out!
If so, here are the steps to follows:

1. Click the More actions icon   ( … ) in the top menu bar.

2. Select Settings.

3. Under the section labeled Open With, select the option for A specific page or pages.

4. Choose Custom from the drop down menu.

5. Enter the web address you want to use as your home page and click the + icon to add that page: you can add as much default home page tabs as you want, repeating this step. 

6. Possibly click the X to the right of an existing home page tab (e.g. about:start) if you want to remove it as one of the default start page tabs.

Step by step procedure to change default page tabs

If you add more default home page tabs, you can also change their order simply dragging in that list (that is right click on the one you want to change the position and then moving – always having the right button pressed – in the position you like in that list, and then release the right button).

So, from now on, when you open the Edge browser, you will always have automatically opened the tabs for all the sites you have defined (possibly more than one), in the order you .

PS: personally I think that the better option is to select Previous pages in the Open with option, so the browser is opened with all the tabs it had the last time it was closed.

That’s all, folks!