Create a link that will open Viber and WhatsApp and will send a message to me

Viber link to user should be like this:

viber://contact?number=%2B0000000000000

Use international telephone number format without + but with %2B.

Since you don't want to create a public account (aka viber links including /pa? ) then simple add the link: <a href="viber://chat?number=012345678901"">chat with me</a> The first two digits are country code and no + sign needed.

What you need is called deep-linking, some examples for viber and whatsapp are:

Viber:

<a href="viber://pa?chatURI=[public account URI]&text=[message text]"> some text </a>

WhatsApp:

<a href="whatsapp://send?abid=[users name]&text=[message text]"> some text </a>

While not a deep link, you can also use the following url (don't use + on phone) for WhatsApp:

<a href="https://api.whatsapp.com/send?phone=0000000">Contact Me</a>

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Sources:

http://origamiengine.com/deep-linking
https://support.viber.com/customer/en/portal/articles/2872423-deep-links