Put the text you want in the menu you will get when you right-click on the map or a feature on the map in MapSource in the field "Menu title".
Next, fill in the URL (the text in the address bar of your web browser) which is used to, for example, open a map at the desired coordinates. With Google Maps this is:
Select the part of the URL you want to replace with one of the variables used by MapSource like the latitude:
Click on the right button on the right side of the input box (in this case "Insert latitude"), and select the right notation in the popup menu (here "Decimal degrees").
The text will change to
Repeat this process for the longitude.
Clicking on the button "Show result" will show you how the menu title, url and wlx file will look like. When you click on "Download wlx file" you will get your wlx file, which you can install in MapSource like this:
Certain variables like the name of the selected map feature can be used in the menu title.
Important to know is that MapSource shows the menu option only when all used variables are present in the selected map feature. Coordinates will always work, but when you used the phone number variable MapSource shows the menu option only when the feature has a phone number attached to it!
You can download some readymade wlx files from this page.