You have to specify the exact option value, meaning you have to know up to three things: The custom ID associated with the email (e.g. for organization-wide addresses), the email being sent from, and the display name. I tried this in my organization, and I came up with a URL that looks like this:
Obviously, my specific details have been left out, but the principle remains the same. Please note that, as always, URL hacking is not supported, and this may change at any time, so code you write today might not be valid in a few months, especially during this time of migration from Classic to Lightning. However, for whatever time is left that this may work, try selecting your drop down, then using the DOM Inspector for your browser to find the appropriate option value, then use that to populate the link. The code will look like this:
<select id="p26" name="p26">
<option value="username@emaildomain:User%20Full%20Name" selected="selected">"User Full Name" <username@emaildomain></option>
<option value="0D2XXXXXXXXXXXX:username2@emaildomain2:Display%20Name">"Display Name" <username2@emaidomain2></option>
You need to use the exact value as specified in the value attribute in order for the appropriate value to be selected.