You need to create a sales order then create a proforma invoice for the sales order.
Create the sales order:
Go to Navigation pane > Modules > Accounts Receivable > Sales orders > All sales orders.
Select New.
In the Customer account field, select the drop-down button to open the lookup.
In the list, find and select the customer record for Contoso Retail Los Angeles.
Select OK.
Under the Sales order lines section, select the Sales order line.
In the Item number field, select the drop-down button to open the lookup.
Select item T0001.
If the Size dimension is not displayed, click on Sales order line > Display > Dimensions.
Select the Size dimension.
Click OK.
In the Size field, select the drop-down button to open the lookup.
Select Size 10.
In the Quantity field, enter 1.
Click on the Sell tab.
Under Generate, select Confirm sales order.
Click OK.
Create the proforma invoice:
On the sales order page, click on the Invoice tab.
Under Generate, select Pro forma invoice.
Set the Print invoice option to Yes.
Click OK.
Click on Export.
Save the invoice in Microsoft Excel format to the Downloads\Sales folder Reference:
https://docs.microsoft.com/en-us/dynamics365/supply-chain/sales-marketing/tasks/create-sales-orders