how to add a signature line to all emails

Hello Rana Machicote,

Thank you for posting this question in Microsoft Community!

As mentioned, how to add a signature line to all your emails. I would like to cooperate with you working on this case. To clarify this case, do you mean that you want to set signature line as default template in Outlook when editing any new emails?

For my suggestion, if you are using Outlook for Windows, please check following Microsoft article as guidance:

Create and add a signature to messages

1. Open a new email message.
2. On the Message menu, select Signature > Signatures.
   Depending on the size of your Outlook window and whether you're composing a new email message or a reply or forward, the Message menu and the Signature button might be in two different locations.

3. Under Select signature to edit, choose New, and in the New Signature dialog box, type a name for the signature.
4. Under Edit signature, compose your signature. You can change fonts, font colors, and sizes, as well as text alignment. If you want to create a more robust signature with bullets, tables, or borders, use Word to format your text, then copy and paste the signature into the Edit signature box. You can also use one of our pre-designed templates for your signature. Download the templates in Word, customize them with your personal information, and then copy and paste them into the Edit signature box. 

Notes:

1. You can add links and images to your email signature, change fonts and colors, and justify the text using the mini formatting bar under Edit signature.
   • You can also add social media icons and links in your signature or customize one of our pre-designed temlates. For more information, see Create a signature from a template.
   • To add images to your signature, see Add a logo or image to your signature.
2. Under Choose default signature, set the following options for your signature:
   In the E-mail account drop-down box, choose an email account to associate with the signature. You can have different signatures for each email account.

If you want your signature added to all new messages by default, in the New messages drop-down box, select one of your signatures. If you don't want to automatically add a signature to new messages, choose (none). This doesn't add a signature to any messages you reply to or forward.
If you want your signature to appear in the messages you reply to and forward, in the  Replies/forwards drop-down, select one of your signatures. Otherwise, accept the default option of (none).

6. Choose OK to save your new signature and return to your message. Outlook doesn't add your new signature to the message you opened in Step 1, even if you chose to apply the signature to all new messages. You'll have to add the signature manually to this one message. All future messages will have the signature added automatically. To add the signature manually, select Signature from the Message menu and then pick the signature you just created.

If you are using Outlook Web, please try the steps as below Microsoft article: Create and add an email signature in Outlook Web App

Hope this would be helpful. I am looking forward to receiving your reply.

Your understanding and patience will be appreciated. Thank you!

https://support.microsoft.com/en-gb/office/create-and-add-a-signature-to-messages-8ee5d4f4-68fd-464a-a1c1-0e1c80bb27f2