To do such a thing I do not believe is supported from the Dynamic Field UI provided by OTRS.
So you can either:
1- add all the possible values into the drop down box and then hide/show them using code changes in the dtl file. (use javascript).
For creating a new ticket there is either AgentTicketEmail.dtl or AgentTicketPhone.dtl.
There is also the CustomerTicketMessage.dtl if you want to include it in the customer interface too.
2- Add only one value which you can also hide using javascript in the dtl files and just add values to the dropdown list using javascript code.
Example javascript below hides/shows different dynamic fields. You can find what your dynamic field is called by looking at the page source from your browser.
function setdynamicviews(){
switch ($('#Dest').val() ) { //this is where the queue is relevant (Dest = Queue)
case "8\|\|Support": // need to slash escape the pipes
//show dynamic fields
document.getElementById('LabelDynamicField_Product').style.display = 'block';
document.getElementById('LabelDynamicField_SerialNo').style.display = 'block';
break;
default:
//hide dynamic fields.
document.getElementById('LabelDynamicField_Product').style.display = 'none';
document.getElementById('LabelDynamicField_SerialNo').style.display = 'none';
}
}
To add items to usign javascript see here
Yuu have not provided enough information for me to help with getting the information "from some distant database"
Note: if you do change any DTL files or other otrs files you should defrinitely create a theme first see here
Hope this helps.