The easiest way is to make it two legs.
Current_Location to Petersburg, VA (City center)
Petersburg (City center) to Myrtle Beach, SC.
You can make them as Custom POIs or save them as Favorites.
You could also start running a Demo on them and stopping the Demo as soon as started. Those 2 destinations now will be in your Recent destinations.
While still on I-95, once past Chester, select the Myrtle Beach location. This will ignore the off and on instructions from Petersbugh city center.
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Since you are using Google maps already to plan your trips, enlarge the map at your "travel to" point so that the correct lane is shown, put your mouse on it right click and the coordinates for the location will be put into the Command box at the top of the map.
Click
example.
For your example I'll pretend you are on I-81 SB and you want VA exit 222.
On your 730 Navigate to ... (or Route A...B if not starting from current device location) > Latitude Longitude and enter the first coordinate from the Google map, preceded by "N" for north.
Tap OK then "W" (west) and enter the second set of numbers from the map, not including the - (minus). Tap OK. Select (.) Fastest route and Done.
If you are not programming a route from the exact current location of the 730 I would recommend you use Route A...B and start at some city center a bit further back but on the route that goes through the place from where you want to start. (I will not try to go backwards, the device will always try to find the shortest route, time wise to destination.
I tried to go from Harrisonburg to Exit 222 but there is no way, unless you specify US250 as the cross road.
As any exit will always be on another road, named or numbered you will have to select the as the Crossroad.