When you go to the Send tab you will see a button 'Create unsigned transaction' at the bottom of all the fields. (Note: you will only see this if this is a watch only setup i.e. a Electrum setup with no seed).
When you select that it will create the transaction which is then saved to a USB drive which you then use on your offline PC/Electrum. Your offline installation is then used to sign that transaction.
So in short:
a) Create the transaction on your watch only installation,
b) Save the unsigned transaction and copy it (via USB drive) to offline installation where its signed,
c) Save signed transaction back to USB drive and move it to your watch only installation,
d) Broadcast it via your watch only installation to the network.
When you select that it will create the transaction which is then saved to a USB drive which you then use on your offline PC/Electrum. Your offline installation is then used to sign that transaction.
So in short:
a) Create the transaction on your watch only installation,
b) Save the unsigned transaction and copy it (via USB drive) to offline installation where its signed,
c) Save signed transaction back to USB drive and move it to your watch only installation,
d) Broadcast it via your watch only installation to the network.
So I need a watch-only, not a regular installation of that. Correct?