You cannot explicitly charge for travel time via the Mable platform. When you are working independently as a contractor, all expenses should be included in your hourly rate. For example: when you hire a plumber, you expect that their hourly rate already includes their expenses such as the travel costs of coming out to your house.
When discussing with a client how much you will charge for your services, make sure to discuss if there will be any transport expected of you. If so, you can negotiate a rate that is slightly higher than your usual rate to account for this. When you send the agreement to your clients, you can incorporate a number of customisable rates. Here’s an example agreement that includes the cost of transport.