When adding a membership to a customers profile, you have the ability to change the membership start date. Doing so will automatically update the first payment total, if the membership is set to bill on a set day of month or week and prorates initial partial periods.
Navigate to the customers membership tab, and click to add membership:
Click on the desired membership you would like to add to the customers profile.
The current start date of the membership, defaults to the current day.
Note that the first payment total is currently €4.81 for this example.
A popup box will appear, where you can choose a new start date. You can either type it in manually, or use the date picker.
Once you have clicked update, the start date will change, and the prorate total for the first payment will be updated.
For this example the start date is now 15th April 2019, and the first payment prorate total is €26.67.