Perhaps something like a Bootstrap modal window?
JavaScript · Bootstrap

Click the "Launch demo modal" button

nothing will annoy your users faster.. than a pop-up.
Not to mention - Ad Blockers, killing it from even being seen.

There are many tools that'll help you create popups like:
Pop up domination
Opt-in monster etc

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The popup it will occur working on the websites it is an spam technique use of this your brand reputation get affected but you need to create popup lot of tools are present wordpress popup,popupally,popuppress.

If you are using WordPress platform then use plugin for pop.

1. Use OptiMonk

2. You can learn to code them yourself or pay someone at fiverr to do them for you.

3. Other cheaper options include: a)GLUE b)POXIT

Hello, Robert.

Using a pop up is a good strategy, so it would be ok to implement it. However, you have to clarify first: What is your main goal? Announcing offers is a means, but, what you want to achieve with it?

Then, think how is your implementation going to affect your user's experience. Based on past experience, I've found out that users feel annoyed with unsolicited content. You'd be killing their interest at once. Of course, you can go all for it and put a popup on your website, or take an alternative way to announce your content to your users and keep them engaged to your website/brand.

My two cents.

Regards,

- Juan José Machado