The Sender ID (the number that the message shows as being sent from on the recipients phone) for outgoing MMS Messages will generally be the number that has been assigned to you by your MMS Gateway. For example when sending messages using the Twilio Gateway messages will appear to be sent from your assigned Twilio number which has been entered into one of the Custom Text fields in the fmMMS Account Details screen. These assigned numbers are typically "long" numbers that can be used to receive replies which can then be stored by the Gateway and downloaded manually or pushed automatically using Webhooks.
You can discuss the available options for Sender IDs with your selected Gateway. Depending on your geographic region and the options provided by the Gateway you might be able to also use Short Codes or alphanumeric sender IDs (N.B. you cannot get replies to an alphanumeric sender ID). Check with your Gateway for further details.