The first method will guide you to customize a quick step that can create a new email message and automatically fill out its subject line with specified text. Please do as following:

1. Shift to the Mail view, and click the Create New in the Quick Steps box on the Home tab.

2. In the popping up Edit Quick Step dialog box,
(1). Enter a new name for your customizing quick step in the Name: box, for example enter Autofill Subject;
(2). Click the Choose an Action box, and select the New Message in the Respond section from the drop down list.

3. Click the text of Show Options below the To box to expand the preset message options, enter your preset subject in the Subject box, and click the Finish button.

Up to now, you have customized a quick part about creating a new message with preset subject. And you can apply this quick part easily with a click.

4. Apply the new created quick part with clicking its name (Autofill Subject) in the Quick Steps box on the Home tab.

Then it creates a new email message , and automatically fills out the subject line with the text you preset in Step 3.

Note: This method is not valid in Outlook 2007 because there is not Quick Steps feature in Outlook 2007 at all.

Automatically add text and datetime to subject or signature for new message in Outlook

Provide by Kutools for Outlook.

Apart from quick steps, you can also create your VBA to automatically fill out subject lines for new email messages in Outlook easily.

1. Press the Alt + F11 to open the Microsoft Visual Basic for Applications window.

2. Insert a module with clicking the Insert > Module.

3. Paste the following VBA code into the module window:

VBA: Create a new email and autofill its subject line

Sub New_Secure_Email()
Dim MItem As MailItem
Set MItem = Application.CreateItem(olMailItem)
MItem.Subject = "your preset subject"
MItem.Display
End Sub

4. Click the Run button on the toolbar or press the F5 key to run this VBA code.

Notes:
(1). In our example, we configure your preset subject as autofill subject. If you need to change, please replace your preset subject with your required text in the VBA code.
(2). This VBA code can be applied in Outlook 2007, 2010, 2013, and 2016.

Then it will create a new email messages and automatically fill out its subject line at once.

Sometimes, it might require to automatically filling current date into subject line when creating new emails. To meet this requirement, you can try Kutools for Outlook.

1. Click the Kutools > Options.

2. In the opening Options dialog box, check the Add date into subject when new email option on the New tab, type a kind of date formatting into below box, and then click the OK button.
Note: In our case, we type the <<Date>> into the box, which will add date as "3/22/2016". You can also type the <<DateTime>> to add date as "3/22/2016 12:21:40 AM".

From now on, current date will be inserted into the subject line automatically when creating new emails. See screen shot below: